Configure database parameters on the right nodes

Since Grizzly, nova-compute does not need database access anymore.
Currently, only nova-api, nova-scheduler and nova-conductor really need
database access.

* Keep original nova parameters with backward compatibility
* Create nova::db with database parameters
* Import nova::db in nova::init for backward compatibility
* Import nova::db in nova::{api,conductor,scheduler}
* Refactorize unit tests for conductor & scheduler

Change-Id: I42b9d2b1efb5856fed6550c25ac3142952690df1
Implements: blueprint move-db-params