[RFR] Add Magento example

![capture decran 2014-05-28 a 16 57 15](https://cloud.githubusercontent.com/assets/2076632/3106425/867f0bb6-e67b-11e3-9106-bdea6e395f63.png)

- [x] Allows to se probe URL for Varnish LB
- [x] Allows to configure `HHVM` `memory_limit` & `max_execution_time` (yes Magento ...)
- [x] Allows to configure `php-fpm` `memory_limit` & `max_execution_time`
- [x] Fix timeout when creation Magento's database
- [x] Retrieve `MySQL` address from env vars
- [x] Use `MySQL` for reading context
- [x] Use `memcached` to store cache tags