env: fix path to glusterfs storage domains

If the hosted engine config specifies a GlusterFS storage domain, look
in the appropriate path on the mounted domain to find it.  The agent
will now use the existing env.path module to get pathnames rather than
having its own (duplicated) code for this.

Change-Id: I7838e373d1cbe0682bfbffa697e6cd5eff819308
Bug-Url: https://bugzilla.redhat.com/1023965
Signed-off-by: Greg Padgett <gpadgett@redhat.com>