Convert roles section into tasks-include_role in deploy-steps.j2.

When executing the deployment_steps_playbook.yaml passing
a specific --tags, it looks like this play gets executed
even though the tag passed in --tags isn't common_roles.

This patch converts the roles: structure into a:
tasks:
  - include_role:

which is the prefered way since Ansible 2.4. This way, the
tags work properly and the execution of both roles is skipped
if the tag doesn't match common_roles.

Change-Id: I772ad486ca11525b8756a0b8cac7a5345373a5d3
Closes-Bug: #1885721
(cherry picked from commit 70eacd3dab)

common/deploy-steps.j2

@@ -481,9 +481,11 @@ outputs:
                   run_once: true
                   debug:
                     msg: Use --start-at-task 'Common roles for TripleO servers' to resume from this task
-              roles:
+              tasks:
-                - tripleo_bootstrap
+                - include_role:
-                - tripleo_ssh_known_hosts
+                    name: tripleo_bootstrap
+                - include_role:
+                    name: tripleo_ssh_known_hosts
               tags:
                 - common_roles