Refactor all boxen arguments into one host ENV variable

[radeksimko]

There's currently a few ENV variables that affect boxen:
https://github.com/radeksimko/vagrant-osx/blob/master/Vagrantfile#L33-35

As this list would only grow and more importantly limit users in what they can change on boxen run, this should be replaced with one variable, e.g. BOXEN_ARGS and have the current set as default.

GH_TOKEN should be probably still kept separately as not many users may want to duplicate all default args just because they want to set the token (which is likely to be a common use-case).