Restructure developing.rst

The "Developing with Keystone" file was incorrectly structured.

1) The "Building the Documentation" section was under the
"dogpile.cache based Key-Value-Store (KVS)" section, which it's
got nothing to do with.

2) The "Running Tests" section was under the "Initial Sample Data"
section, which it's got nothing to do with.

3) Several testing-related sections could be put under their own
"Testing" section to keep them together.

Change-Id: Icb0c63385d9ccc24b91d17e057e60346ce1a648e
Merge "Restructure developing.rst"