Convert Savanna to an extras-style setup

The new extras.d-based hook system provides a simple way to add
additional projects to DevStack's processing without modifying
the core scripts.

Change-Id: Ib1ab873783a80acec771bf5e90126042727c28e3
Merge branch 'fix-get-packages' into dt-test
Merge branch '50601-savanna' into dt-test
Merge branch 'extras' into dt-test
Merge branch 'ldap-ad' into dt-test