mgr/volumes: check for string values in uid/gid

mgr/volumes: check for string values in uid/gid

chown allows strings as per bbbfb44453f204286a8ff312349d42afde5fced6,
which caused this error. Eventhough uid/gid are input as CephInt, the
qa tests can pass in only string values to _fs_cmd. So try converting
the incoming uid/gid to int in create_subvolume and create_group.
It might be a valid string.

Fixes: https://tracker.ceph.com/issues/43038
Signed-off-by: Jos Collin <jcollin@redhat.com>
(cherry picked from commit 4c1029bc083b44d87f01f4e503d3ad64a568df52)