Some environment changes to speed up deploys

This is so that we can have multiple supervisors running
in different virtual envs

adding another push queue

* Instead of using pypi.edx.org why not cache on localhost?
  Seems to work well and it does result in a speed improvement
  for when we need to download packages. This adds the devpi
  role as dependency for edxapp
* For edxapp we md5sum all fo the requirements files and only
  run the pip installs, bundle, npm tasks if any one of those
  files have changed (it treats them as a group)
* Adds convenience symlinks for pip

Use the EDXAPP_LANG var which already exists.

Add github.com to ssh known_hosts for jenkins

fixing check for cluster config

Change the default cloudformation elb backend ports.

mongo_clustered defaults to !!null

Now they default to the same as what is set in ansible which should reduce the number of things
we have to override when running ansible on a VPC.

Update to use cluster enabling vars in both rabbit and mongo.

adding reconfigure option for

Move xqueue to supervisor

http://jenkins.edx.org:8080/view/Ansible/job/ansible-deploy

To ensure they are taken into account before we try to start the process
through supervisord.

supervisord as well as all the processes it controls are run as
`{{ common_web_user }}`. This allows us to connect to it as that
user which allows us to manage the processes as www-data, instead
of root.

Common refactor - Fix ora & xqueue

Feanil/mongo rs

Use the mongo path settings provided by the mongo role.

Jarv/xserver updates