qa/tasks/ceph_manager: make fix_pgp_num behave when no pool is found

qa/tasks/ceph_manager: make fix_pgp_num behave when no pool is found

Fixes:

2020-01-30T04:41:24.697 INFO:tasks.thrashosds.thrasher:fixing pg num pool None
2020-01-30T04:41:24.698 INFO:tasks.thrashosds.thrasher:Traceback (most recent call last):
  File "/home/teuthworker/src/git.ceph.com_ceph-c_wip-sage-testing-2020-01-29-1034/qa/tasks/ceph_manager.py", line 1070, in wrapper
    return func(self)
  File "/home/teuthworker/src/git.ceph.com_ceph-c_wip-sage-testing-2020-01-29-1034/qa/tasks/ceph_manager.py", line 1200, in _do_thrash
    self.choose_action()()
  File "/home/teuthworker/src/git.ceph.com_ceph-c_wip-sage-testing-2020-01-29-1034/qa/tasks/ceph_manager.py", line 768, in fix_pgp_num
    if self.ceph_manager.set_pool_pgpnum(pool, force):
  File "/home/teuthworker/src/git.ceph.com_ceph-c_wip-sage-testing-2020-01-29-1034/qa/tasks/ceph_manager.py", line 2088, in set_pool_pgpnum
    assert isinstance(pool_name, six.string_types)
AssertionError

Signed-off-by: Sage Weil <sage@redhat.com>