Decouple FCM and APNs proxies from same project

In this PR the push-network-proxies repository was refactored in order to allow adding additional proxies and OpenShift templates. The whole old project was moved to folder apns-java but it is necessary to extract all GCM/FCM functionality from there and put it under `gcm-java` folder. This way both proxies will be completely independent.

The importance of maintaining the proxies repository in good shape resides in the need of testing the push functionalities, specially performance.

https://github.com/aerogear/push-network-proxies