Clean up imports

Remove a bunch of unused imports and grouped/ordered the existing
ones better.

Fixes OS-226

Change-Id: I2f9f14ef70a03f9d48ab371be1f81dbe53b7e38f
Merge branch 'grizzly/1.5' into havana/1.5

Conflicts:
	cobalt/nova/api.py
	cobalt/nova/extension/manager.py

Change-Id: I561dba87b349a1118d54856d1450e6421ef975d5