qa/tasks/ceph.py: populate mnt_point in hammer

qa/tasks/ceph.py: populate mnt_point in hammer

In hammer, there is no "mnt_point" variable, yet 53225d5 was cherry-picked to
hammer - see 87e0299448908775600c62130617bad503027768 and https://github.com/ceph/ceph/pull/12455

This caused lots of "NameError: global name 'mnt_point' is not defined" errors
in hammer integration testing.

This is not a cherry-pick, because the mnt_point variable was introduced by
a54ff597faa379c8fd369352fb924651be269f95 ("tasks/ceph: convert cluster creation
to work with multiple clusters") which was not backported to hammer.

Fixes: http://tracker.ceph.com/issues/18383
Signed-off-by: Nathan Cutler <ncutler@suse.com>