mgr/volumes: return string type to ceph-manager

mgr/volumes: return string type to ceph-manager

The 'volumes' ceph-mgr module returns a byte type instead of a string
type as the stdout result of commands, `fs subvolume getpath` and
`fs subvolumegroup getpath` to the ceph-mgr. This causes the commands
to hang and the ceph-mgr to utilize 100% CPU. Fix the 'volumes'
ceph-mgr module to return string type to ceph-mgr as the stdout
result of the commands.

Introduced-by: 3d63cd947f55a8508f972ee249ea6a04836429ae
Fixes: https://tracker.ceph.com/issues/41903
Signed-off-by: Venky Shankar <vshankar@redhat.com>