Refactor init_nova() to simplify Grenade's upgrade-nova

* Splits out create_nova_cache_dir() and create_nova_keys_dir()
* Random tidy-up changes

Change-Id: I20d995d4c2e5facfb912ee03a6cda6c56f20bbe9
Merge branch 'nova-config-cleanup' into dt-test
Merge branch 'database-service' into dt-test