mgr/volumes: fix py2 compat issue

mgr/volumes: fix py2 compat issue

Fix the following issue seen while upstream teuthology testing,
 File "/usr/share/ceph/mgr/volumes/fs/operations/versions/subvolume_base.py", line 98, in load_config
   self.metadata_mgr = MetadataManager(self.fs, self.legacy_config_path, 0o640)
 File "/usr/share/ceph/mgr/volumes/fs/operations/versions/subvolume_base.py", line 73, in legacy_config_path
   meta_config = "{0}.meta".format(m.digest().hex())
 AttributeError: 'str' object has no attribute 'hex'

This issue is not observed in master/octopus, as it only supports
py3.

Signed-off-by: Ramana Raja <rraja@redhat.com>