Move static collection into its own task file.

This also makes it easier to conditionalize it since it
doesn't need to be run on workers.
Add support for worker upstart scripts.
Add a section for the workers to the top level playbook.
Conditionally install workers and start them.
Move worker upstarts to their own files.
Remove endif.
Put the workers on the edxapp machines and into the sandbox.
adding rabbitmq to the sandbox play (needed for workers)
parameterizing edx-workers.conf on service_variants_enabled
Merge pull request #164 from edx/feanil/ansible_workers_rebase

Feanil/ansible workers rebase