Introduce workarounds to foreman playbook

Adds workarounds support to the foreman playbook:
* Add new pre/post playbooks specific to foreman
* Add new group_by to workarounds-prep
* Use the new group to apply workaround just to RHEL-7
* Move some workarounds out of the openstack-installer role into their
  own role
* Remove the neutron password fix. That was an extremely temporary
  workaround

Change-Id: If2a5a8c632fe069f6456c02fe8df483b8e2e5fc2