task/cifs-mount.py: Task for mounting cifs

The cifs-mount task mounts a smb endpoint from the
first available smbd server (the samba.0 role).  This
task is similar to the ceph-fuse task, file system
tests can be run on the resulting mount point.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
task/workunit: Add mnt-prefix option as test loc

Normally the workunit task creates a scratch directory within the ceph
mountpoint (<testdir>/mnt.0), changes to that directory, and runs the
tests.  In some cases (cifs) we need to use a separate mnt prefix, so
we allow specifying the prefix with the mnt-prefix option.

Signed-off-by: Sam Lang <sam.lang@inktank.com>