sandbox-ora: Include xqueue & ORA on the `edx_sandbox` playbook

Includes modifications to allow to run on the xqueue & ORA services on the same instance as the LMS & CMS, to fit them all into the sandbox VM.

* Switches to use the `virtualenv` role as suggested by @feanil in https://github.com/edx/configuration/pull/207
* Uses its own user to prevent the `virtualenv` role from attempting to modify the user `www-data`, which fails
* Adds handling of the `SERVICE_VARIANT` variable by xqueue and ORA (cf pull request https://github.com/edx/xqueue/pull/65 and https://github.com/edx/edx-ora/pull/136 ), which works the same way as for the LMS/CMS services, and allows to use a non-default filename for the env/auth configuration JSON files. Support for service variants is more than what is strictly needed here, but I thought it would be useful to maintain the configuration file naming mechanisms consistent across services.
* Allowed to configure the binding IP of rabbitmq, which by default binds on the main IP, while the default configuration of the other services expects localhost
* Added creation of edx/edx user in the default configuration of rabbitmq, to allow Xqueue to connect
* Fixed various issues with the default configuration values for ORA & Xqueue: missing '/' in config, 'NaN' printed in configuration files when ansible can't guess the number of cores, HTTP instead of HTTPS, etc.

You will need to set the following ansible variables to use the modified versions of ora and xqueue:
```
xqueue_source_repo: https://github.com/antoviaque/xqueue.git
ora_source_repo: https://github.com/antoviaque/edx-ora.git
```