Add optional regexp validation to service dialogs

Text boxes now expose a drop-down for validator type and
a text field for validator rule. When 'Regular Expression'
is selected for the validator type, user input will be
matched against the provided validator rule. This is similar
to how Foreman validates user input for Smart Variables.

https://bugzilla.redhat.com/show_bug.cgi?id=1085158
@dillaman We do not require specs for simple migrations like this as it is just testing rails functionality which is already tested in the rails framework.  We require migration specs for more complex migrations when existing data is involved.