qa: avoid explicit set to client mountpoint as "/"

qa: avoid explicit set to client mountpoint as "/"

This causes self.cephfs_mntpt to set as "/" by default which
overrides the config in ceph.conf. `test_client_cache_size`
updates ceph.conf with:

        client mountpoint = /subdir

However, the ceph-fuse mount command has --client_mountpoint explicitly
set as "/", thereby causing the root of the file system to get mounted which
confuses the test.

Fixes: http://tracker.ceph.com/issues/56446
Introduced-by: bf83eaa4e75516a6937e4097b8708c48856a9473
Signed-off-by: Venky Shankar <vshankar@redhat.com>
(cherry picked from commit 4322dcc2e94bab80042eaf1e236174f6e6772cec)