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 (cf pull request https://github.com/edx/xqueue/pull/65), 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.