Add environment to disable deploy steps

Add environment to disable deploy steps

This enables either deploying without configuring any services, or
temporarily disabling the deploy steps such as will be required
for minor updates where we want to re-run the rolling update outside
of heat.

To deploy directly via ansible-playbook you can do e.g:

openstack overcloud config download --config-dir tmpconfig
cd tmpconfig/tripleo-6b02U7-config
ansible-playbook -vvv -b -i /usr/bin/tripleo-ansible-inventory deploy_steps_playbook.yaml

Which will run the same ansible steps as we normally run via heat.

Change-Id: I59947b67523dfcc43d454d4ac7d82b06804cf71d