Add library search_env_filtered

Provides a search that automatically applies an "env_filter", i.e. doing a search against the correct proposal. To make this work, nova/db/database_instance had to move to nova/database_instance (in line with _all_ other *_instance entries in JSON files).

As a nice side effect, most previously broken env_filter get fixed automatically. Before, some search() invocations re-(ab)used env_filters from previous searches for entirely different barclamps. This should help re-supporting multi-proposal in the future.

This approach should work for other barclamps too, but with Crowbar's current architecture, each would have to ship it's own copy of this library. A follow-up commit could move this to a more appropriate (shared) location.