Add  guest_account to parameters, maps to 'guest account'

Thanks for the Samba module, really helpful to me as a lot of Developers don't consider Windows hosts machines when working with Vagrant.

I find it useful to be able to use Samba 'guest account' to change the guest user, particularly when using Puppet to provision a guest machine with Vagrant. The default guest user 'nobody' can cause problems with permissions. I have added $guest_account parameter.