qa/cephfs: allow reusing mount objects and add remount method

qa/cephfs: allow reusing mount objects and add remount method

This commit introduces following two set of changes -

First, make client keyring path, mountpoint on host FS and CephFS and
CephFS's name attributes of the object representing the mount
and update all the mount object creation calls accordingly. Also,
rewrite all the mount object creation to use keyword arguments instead
of positional arguments to avoid mistakes, especially since a new
argument was added in this commit.

Second, add remount method to mount.py so that it's possible to unmount
safely, modify the attributes of the object representing the mount and
mount again based on new state of the object *in a single call*. The
method is placed in mount.py to avoid duplication.

This change has two leads to two more changes: upgrading interface of
mount() and mount_wait() and upgrading testsuites to adapt to these
change.

Signed-off-by: Rishabh Dave <ridave@redhat.com>