User Guide: Add CLI cheat sheet

Add a short CLI cheat sheet.

Based on the cheat sheet at
http://anystacker.com/2014/02/openstack-command-line-cheat-sheet/

Change-Id: I7e90377bab64cbfbd32ca83c40b48851c78e7dad
Co-Authored-By: Egle Sigler <ushnishtha@hotmail.com>
Closes-Bug: #1317531
Merge "User Guide: Add CLI cheat sheet"