Update Vagrantfile to set configuration based on current provider

When implemented like this, the users don't need to specify
configuration each time they run vagrant commands. The provider
should be specified only once, during 'vagrant up' and only if
it is not Virtualbox.

Most of users will need to just do 'vagrant up' henceforth.
Merge pull request #261 from racktear/master

Update Vagrantfile to set configuration based on current provider