Add customisable OS to Vagrantfile

Fixes #1674 
Fixes #1691
Fixes #1692

Choose the OS you want to run with the `ALAVETELI_VAGRANT_OS` environment
variable.

**Confirmed working* under:**

- [x] `precise64`
- [x] `squeeze64`
- [x] `wheezy64`

*  *"Working" being defined as getting a `200` code when curling the homepage from the host machine. 9e464e6 introduces a script to automate this testing.

Examples:

    # Use default (Ubuntu 12.04 LTS (precise)
    vagrant up

    # Use Debian (7) Wheezy
    ALAVETELI_VAGRANT_OS=wheezy64 vagrant up

<!---
@huboard:{"order":1717.0,"milestone_order":1720,"custom_state":""}
-->