ceph-volume: tests shouldn't require lvm2 to pass

ceph-volume: tests shouldn't require lvm2 to pass

some tests are currently failing when `lvm2` isn't installed:

```
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestList::test_empty_device_json_zero_exit_status - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestList::test_empty_device_zero_exit_status - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestFullReport::test_no_ceph_lvs - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestFullReport::test_ceph_data_lv_reported - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestFullReport::test_ceph_journal_lv_reported - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestFullReport::test_ceph_wal_lv_reported - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestFullReport::test_physical_2nd_device_gets_reported[journal] - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestFullReport::test_physical_2nd_device_gets_reported[db] - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestFullReport::test_physical_2nd_device_gets_reported[wal] - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestSingleReport::test_not_a_ceph_lv - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestSingleReport::test_report_a_ceph_lv - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestSingleReport::test_report_a_ceph_journal_device - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestSingleReport::test_report_by_osd_id_for_just_block_dev - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestSingleReport::test_report_by_osd_id_for_just_data_dev - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestSingleReport::test_report_by_osd_id_for_just_block_wal_and_db_dev - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestSingleReport::test_report_by_osd_id_for_data_and_journal_dev - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestSingleReport::test_report_by_nonexistent_osd_id - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_listing.py::TestSingleReport::test_report_a_ceph_lv_with_no_matching_devices - FileNotFoundError: [Errno 2] No such file or directory: 'pvs'
FAILED ceph_volume/tests/devices/lvm/test_migrate.py::TestNew::test_newdb_not_target_lvm - FileNotFoundError: [Errno 2] No such file or directory: 'lvs'
FAILED ceph_volume/tests/devices/lvm/test_zap.py::TestEnsureAssociatedLVs::test_nothing_is_found - FileNotFoundError: [Errno 2] No such file or directory: 'lvs'
FAILED ceph_volume/tests/devices/lvm/test_zap.py::TestEnsureAssociatedLVs::test_multiple_journals_are_found - FileNotFoundError: [Errno 2] No such file or directory: 'lvs'
FAILED ceph_volume/tests/devices/lvm/test_zap.py::TestEnsureAssociatedLVs::test_multiple_dbs_are_found - FileNotFoundError: [Errno 2] No such file or directory: 'lvs'
FAILED ceph_volume/tests/devices/lvm/test_zap.py::TestEnsureAssociatedLVs::test_multiple_wals_are_found - FileNotFoundError: [Errno 2] No such file or directory: 'lvs'
FAILED ceph_volume/tests/devices/lvm/test_zap.py::TestEnsureAssociatedLVs::test_multiple_backing_devs_are_found - FileNotFoundError: [Errno 2] No such file or directory: 'lvs'
FAILED ceph_volume/tests/objectstore/test_lvmbluestore.py::TestLvmBlueStore::test_activate_all_osd_is_active - FileNotFoundError: [Errno 2] No such file or directory: 'lvs'
```

Everything should be actually mocked. This commit addresses that.

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>