rename ExtensionsResource to ExtensionsController

every other "resource" in the api is named either *Controller or just
Controller, so updating this one to match, no effect on anything.

here's a little audit of just about all the nova api calls, the stuff at
the bottom shows some stats:
https://gist.githubusercontent.com/termie/9500927/raw/ac724f0762224bc60bb79c4b0fecfdd12afe5ca7/calls

Change-Id: I71df08048d5b0fa868f68bfe9cb9176507dde3b7
Merge "rename ExtensionsResource to ExtensionsController"