Add support for setting up ansible-galaxy on the controller node

[jbonisteel]

This is part of the work being done in: #27

It adds support for ansible-galaxy so we can use the sshd_user galaxy module to unify sshd configuration and ensure all users have access to all hosts

The missing pieces/next steps should be:

• checking that the ansible-galaxy setup on the controller node actually works
• Adding the ansible-galaxy module as dep to the ssh_users role
• Checking it applies to the hosts we mentioned above
• Applying it to all hosts

Corresponding PR: #75