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
Merge pull request #14 from dillaman/bz1085158-service-dialog-regex-validation

Add optional regexp validation to service dialogs