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
Add --catalog to service show

Shows endpoints from the service catalog rather than the system services.

Change-Id: I842916af9f7c0a76c4d3e27e419bf0fec059ec78
Merge "Add show limits command"
Merge "Add --catalog to service show"