Configure the cache layers with the correct gridset.

* But how to create the right gridset to begin with?
Added geowebcache.xml with custom gridSets.

* Alias 900913 to 3785
Updated cloudformation to use 2 pip installs.

* The first pip install sets up django, etc
* Second install is expensive, and installs PySAL. This is run in the background
  and joined later in the install process.