Added a OGC_Servers convenience class.

The good:
* Removes the OPTIONS dict.  The OPTIONS dict in the DATABASES setting in Django is used to pass kwargs to the db backend, which is not how we were using it.
* We can store commonly used logic as methods on the OGC_Server class.
* Attributes on OGC_Server instances are translated into get() calls to the server dict which avoid KeyErrors and common if statements.
* Will support mulitiple servers.

The bad:
* No KeyErrors can lead to mistakes.
* Another change to our settings.py (which touches 20 files).
* Tests are in geonode/base/tests.py, the logic is in geonode/utils.py
* Values are called with mix case: ogc_server_settings.LOCATION.