Change array_string_field and add :only_comma option as true

This is fix for bnc#903674
Crowbar and/or nova cannot cope with vCenter cluster names containing spaces
https://bugzilla.suse.com/show_bug.cgi?id=903674
Add quotation marks to vmware cluster_names option
Merge pull request #404 from mjura/bnc903674

Change array_string_field and add :only_comma option as true