Vagrantfile update for Multi VM support

[humblearner]

Reference: #67

[humblearner]

Got this error with u14 bento box couple of times:

TASK [ANXS.postgresql : PostgreSQL | Make sure the PostgreSQL users are present] ***
An exception occurred during task execution. To see the full traceback, use -vvv. The error was: UnicodeEncodeError: 'ascii' codec can't encode character u'\u2018' in position 29: ordinal not in range(128)
fatal: [u14]: FAILED! => {"failed": true, "msg": "Unexpected failure during module execution.", "stdout": ""}

[arm4b]

We can define box for every VM in VIRTUAL_MACHINES constant, what will allow us to simplify this block:

vm_config.vm.box = cfg[:box]

[arm4b]

u14 is assumption we made internally, but for other users of this repo and contributors it could be confusing.
I think vagrant up ubuntu14 is more expected here.

[arm4b]

I left a couple of small change requests.

Besides of that, looks good.

[arm4b]

Looks really nice 👍