Add dockerfiles for keystone fernet

This adds the docker aspects of fernet key bootstrapping as well as
distributed key rotation.

- Bootstrapping is handled in the same way as keystone bootstrap.
- A new keystone-fernet and keystone-ssh container is created to allow
  the nodes to communicate with each other (taken from nova-ssh).
- The keystone-fernet is a keystone container with crontab installed.
  This will handle key rotations through keystone-manage and trigger
  an rsync to push new tokens to other nodes.

The Ansible component is implemented in:
  https://review.openstack.org/#/c/349366

Change-Id: Id610e00e8c63c7f1bc0974c0aa1b3f44c18e1019
Partially-Implements: blueprint keystone-fernet-token
Partially-Implements: blueprint third-party-plugin-support