Initial set of files to bring up a staging edx

The main thing of interest here is stage-ansible.cfg and
stage-ssh-config, which taken together offer a way to have an
environment with different access control policies and a different VPC
jumpbox without disturbing the usual case of working with your VPC
production hosts.

To run this, edit stage-ssh-config with a sensible jumpbox configuration
and make sure the options in stage-ansible.cfg are the set of ansible
options you expect to use, then invoke ansible with ANSIBLE_CONFIG like
so:
```
ANSIBLE_CONFIG=stage-ansible.cfg ansible-playbook -v -i './ec2.py' edxapp_stage.yml
```
Merge pull request #125 from edx/feature/jrbl/staging_vars

Initial set of files to bring up a staging edx