Performance optimization for contrib.flavorextraspecs

In method FlavorExtraSpecsController.index():
This method calls FlavorExtraSpecsController._get_extra_specs()
that returns all extra_specs that has flavor.
In db.api layer we are returning already extra_specs as plain object,
so there is no reason to make one more time copy of extra_specs.

In method FlavorExtraSpecsController.show():
This method should return extra_spec that corresponds to specified
key and flavor_id.
Was:
Get all extra_specs by flavor_id and filter by key locally
Now:
Filter by flavor_id and key in db.

Change-Id: I4c39f5f56511e67e37482cda1b4e9771ee9c7d7d
Merge "Performance optimization for contrib.flavorextraspecs"