AGPUSH-997: make iOSVariant.production required parameter on UI
AGPUSH-997: provide meaningful messages to all constrainst in Variants

Conflicts:
	model/api/src/main/java/org/jboss/aerogear/unifiedpush/api/WindowsWNSVariant.java
AGPUSH-997: provide meaningful message when APNS certificate is not valid
added certificate bean validation test