Allow empty config files.

[thom8]

Currently if a config files exists but has no YAML vars defined vagrant will error.

[Decipher]

Regression error, changing back to dir which is not set will cause this Vagrantfile to break.

[thom8]

Updated to use cwd

[Decipher]

Code looks good, will have to run a few manual tests to confirm for sure though.

[Decipher]

Missed a change on line 23; Still references vagrant_config and therefore the Vagrantfile fails.

[Decipher]

Should also make these changes to the box Vagrantfile.

[thom8]

Updated box Vagrantfile - having both of these in the project will make it really difficult/confusing for any new contributors, we should look at tidying it up ASAP -- #94

The Vagrantfile's should be exactly the same except for the skip conditions for the box version.

[Decipher]

This PR doesn't 100% resolve the issue, due to the definition of 'empty'.

If I have a .beetbox/config.yml file with absolutely nothing in the file it does not work, it throws the following error:

There was an error loading a Vagrantfile. The file being loaded
and the error message are shown below. This is usually caused by

a syntax error.

Path: /Users/stuartclark/Documents/Projects/beetbox/Vagrantfile
Line number: 31
Message: TypeError: no implicit conversion of false into Hash

However, if the .beetbox/config.yml file has --- in the file it does work.

I assume the completely empty file is not a valid YAML file, but it seems more likely to be the case than a valid empty YAML config file.