Enable external APNs push notification configuration

By default, UPS should use reasonable default setup (URL, no proxy, etc.) for push notification networks it supports (GCM, APNS, MPNS, etc.), however it should be possible to alter these value via external configuration.

This allows user to easily set a proxy or point notification to use URL of a testing service instead.

Implementation note: all external configuration, including AGPUSH-1149 can be in a single JSON file.