qa/cephfs: install ceph-mgr-modules-standard for cephfs tests

qa/cephfs: install ceph-mgr-modules-standard for cephfs tests

Now that the ceph-mgr plugins are being separated into essential and
non-essential packages (always-on vs. optional), cephfs qa suite
requires the optional packages for ceph-mgr plugins which are not
always-on, but are being tested with fs suite. The good thing is, we
can install _all_ optional plugins using ceph-mgr-modules-standard
package instead of installing cherry-picked packages.

Signed-off-by: Venky Shankar <vshankar@redhat.com>