Add new role for a server which runs migrations and collects static files

Adding this as a pull request to verify the logic with you @seanherron 

We'll tag one of the current pay.gov servers with "mutexleader" (instead of "paygov"). Whenever a deployment occurs, the mutexleader will migrate tables and collect static files. This removes migrations from the "paygov" role, as they are no longer needed there, but keeps "collectstatic"ing in the webservers until their settings files have been adjusted.

Next step is to modify the settings files to point to the correct ec2 buckets.

Something that'd be nice to add: Hypothetically, a server configuration could have no mutexleader. We should give a warning or similar, somewhere.