qa/cephfs: improvements for name generators in test_volumes.py

qa/cephfs: improvements for name generators in test_volumes.py

Generate a name that is shorter and easier to remember.

Also, write a simpler, faster & better helper method for generating
unique names. This method will also have shorter and more concise name,
so this will be easier to type and easier to read.

Fixes: https://tracker.ceph.com/issues/63680
Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit ad96e91ee4f6d1bcd47cfd77725d27d64ee06c93)

Conflicts:
	qa/tasks/cephfs/test_volumes.py