Mark the service as readonly in the UI in authpam plugin

Update the Option class to take a readonly keyword argument,
defaulting to False. Extend its subclasses to pass this value
along.

The page template will add the disabled keyword to input and
textarea if a config option is marked as readonly.

https://fedorahosted.org/ipsilon/ticket/6

Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>