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.