Add show limits command

* This is a combination of the compute and volume API limits as they are
  very similar.  As such, the command lives in a new command group
  'openstack.common' that is unversioned.
* Implements 'limits show [--absolute|--rate]

Updated for https://review.openstack.org/#/c/36772/

Bug: 1172057

Change-Id: I2bd181cd0d098f7143360ae67944c2f221379af5
Merge "Add show limits command"