cephfs-shell: Fix 'lls' command errors

cephfs-shell: Fix 'lls' command errors

This patch fixes following:
* Not printing complete path when '..' is passed as argument.
* Printing of path for current working directory.
* No need to set default value for add_argument. As it returns empty list with
  nargs='*'.
* No need to check for absolute path. os.listdir takes both relative and
  absolute path.
* Use try-catch instead of if-else statement to catch exceptions.

Fixes: http://tracker.ceph.com/issues/40244
Signed-off-by: Varsha Rao <varao@redhat.com>