fixes #2975 added setting for using the shortname instead of FQDN for new VMs

Hello!

I made a setting for using the shortname when creating new machines. Unfortunately our test vsphere cluster is currently down so I cannot test it. 

If someone tries to provision a hostname twice the error from the virtualization should be passed on, on which Foreman refuses the creation. I therefore haven't implemented any error handling (hope I'm right :) ).

Cheers
Hannes