(cherry picked from commit 9d3475d4)

Installs python 2.7 using raw ansible commands.

`python` role is prepended to edx_sandbox.yml and edx-stateless.yml in a
prefix playbook, so that gather_facts can be "no".

Xenial browser requirements

* Remove OS version conditionals
* Pin Firefox to v45.*

* No need to download our specific version
* Use officially supplied FF 45

boxcutter/ubuntu1604 needs recent Vagrant, so pin at least 1.8.7

Xenial doesn't have Python 2 that ansible needs, so use ansible-bootstrap.sh to get it, though that might be way more than we need.

remove systemctl calls to release dpkg lock

We use to constrict our builds to PRs, similar to edx-platform

temporarily install google chrome stable

disable auto apt updates in xenial

Update credentials playbook

add fb and tw accounts to generic_env_config

Adds COMMON_EDXAPP_SETTINGS

* Default value is 'aws' (unchanged)
* Used by EDXAPP_SETTINGS, gitreload_env.DJANGO_SETTINGS, and demo_edxapp_settings variables

Conditionally adds postfix_queue role to edx_sandbox.yml

ECOM-5776

Ready for deploying Mongo 3.2