Docs: add information how to install EAP7 via RHN channels

As we want to provide EAP7 RPM packages also via RHN network in a very near future, we need to update our documentation appropriately. This applies mainly to RPM Installation section in Installation Guide though.

Things that need to be mentioned/added:

1. mention that apart from current CDN RPM source, there is a RHN source also
2. names of RHN channels (only 'minor' channels have been created so far - no 'current' channels will be created):

   jbappplatform-7.0-i386-server-6-rpm
   jbappplatform-7.0-x86_64-server-6-rpm
   jbappplatform-7.0-x86_64-server-7-rpm
   

3. steps to register those channels on the machine are (similar to EAP6.4)
4. note - for RHEL7 the Red Hat Network Classic is not supported anymore, only when using Red Hat Satellite 5.7+ , info got from here
5. all following steps for installation should be same as they are already documented for CDN via subscription-manager though