Changes to unify edxapp and worker AMIs.

Always write the supervisor files and link them as necessary.

Drop the configs in the supervisor available directory. Instead of having a
seperate cfg dir for each service.

Seperate devstack logic from disabling supervisor tasks. Add the new flag so
devstack continues to not startup supervisor. Use new supervisor_available_dir
everywhere.

Remove mongo logic from abbey and remove stage_release. This should completely
remove our tie ins to mongo.

Also removing monitor repos script which depended on stage_release. We can add
in similar logic when we're ready to automate building the AMIs on rc branch
creation.

Updated requirements.