Create ability for scripts to be run once or always on every vagrant up

While there is "exec-once" and "exec-always," if a user were to want to run a script on every start up or just once, they would need to re-provision every time.
I'm suggesting making a secondary execute-files.sh that would be executed on every vagrant up to allow for certain scripts to be run if found in a "startup-once" and "startup-always" folder.
It would just be added as an additional provision script in the vagrantfile with :run => "always" set on it.

Adds functionality described in #812