qa/cephfs: don't expect "file exists" error for "mkdir -p"

qa/cephfs: don't expect "file exists" error for "mkdir -p"

Get rid of try-except block in _create_mntpt() since "mkdir -p" doesn't
raise any error when the directory to be created already exists. Also,
use chmod command instead of mkdir command to set the permission mode on
directory since mkdir command would have no effect on the directory's
permission mode if the directory already exists.

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