Restructure playbooks for multiple commands

Much of the logic in the existing 'deploy' playbooks could be reused for
a teardown command, so it didn't make sense to duplicate this. Instead,
rename/restructure playbooks such that they refer to an area of
responsibility, rather than an action.

Also, make some changes to allow a new 'teardown' action.

ansible/deploy.yml

@@ -1,12 +0,0 @@
----
-- name: Perform deployment host configuration
-  import_playbook: deploy_hosts.yml
-
-- name: Perform deployment node configuration
-  import_playbook: deploy_nodes.yml
-
-- name: Enrol nodes in Ironic
-  import_playbook: enrol_nodes.yml
-
-- name: Register flavors in Nova
-  import_playbook: register_flavors.yml