sync the locust role with edx-load-tests

ECOM-7207

* Do not start locust automatically, instead make the locust user
  suitable for interactive manipulation and execution of the codebase.
  This works well for the current iteration of edx-load-tests which
  expects the user to hand-modify settings files.

* For convenience, the locust user gets dropped straight into the
  edx-load-tests codebase and sources a virtualenv with most
  dependencies already installed.

* Add an extra MOTD related to loadtesting

PERF-415

Improve docs and become www-data

I see no reason for us not to retry

The continuous-deliver/ playbooks are already doing this.

mattdrayer/ENT-244: Add ENTERPRISE_SERVICE_URL setting

mattdrayer: Remove ENTERPRISE_API_URL setting

Change the default XQUEUE_RABBITMQ_TLS value to false

We didn't need this previously, so I'm a little concerned by it.
Tweaked as part of DEVOPS-5543

rabbitmq memory alerting

Make prior RabbitMQ TLS changelog more accurate, and add a new entry for setting `XQUEUE_RABBITMQ_TLS` to false

Don't truncate version shas when tagging AWS resources.

update mongodb version and repo

It's hard to know when a repo will get big enough that any given
truncation won't be long enough.

Closes #3717.

update mysqlclientcore for 16.04

Put the password generator into a real .sh file

There were two whens in these tasks

Ansible warned at the top, rather than complaining louder.

add the ec2 plugin + deps

Ansible Script to rollback migrations

turn off mongo auth for platform jenkins

Update Vagrantfiles for ficus.1

(cherry picked from commit bc971b0f)

Source python virtual env in analytics_api_env

https://wiki.jenkins-ci.org/display/JENKINS/Amazon+EC2+Plugin

Enable Mongo auth