Racks CRUD commands

Commands implemented previously and tweaked:
rack-show
rack-list

Newly implemented commands:
rack-create
rack-update
rack-delete

We might eventually want to provide a way to set capacities for racks
and flavors one by one, but after discussing with multiple people we
still don't have obvious correct way to do this, so i'm leaving it out
for now and it can be added later. Currently we can set capacities in
bulk via --capacities.

Partially implements
https://blueprints.launchpad.net/python-tuskarclient/+spec/tripleo-racks-cli

Change-Id: Ic45f3c157e56b976438c9559245858c831511797