ceph-volume: remove class Volumes from api/lvm.py

ceph-volume: remove class Volumes from api/lvm.py

And remove methods is_lv(), get_lv() and get_lv_from_argument() since
these methods uses class Volumes and get_first_lv() or get_lv() can be
easily used instead when needed in future.

Also, update tests and comments accordingly.

Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit e09d3cd1a5bcfb42972b6958fb7e6c42a645549d)

 Conflicts:
	src/ceph-volume/ceph_volume/tests/util/test_device.py
	Test "test_is_member_lsblk" has a slightly different signature
	than master.