Fix help of some optional arguments

Some positional arguments are optional but the help indicate
they are mendatory and can be confuse for an user.

  swift delete <container> [object]
  swift post <container> [object]

Change-Id: I7be525905b51d3ef160b0f47194e817b5cba0f9b
Merge "Fix help of some optional arguments"
Merge branch 'master' of ssh://review.openstack.org:29418/openstack/python-swiftclient