Core: Added possibility to run filter_on with multiple attrs

So instead of writing:

instances = filter_on instances, :id, opts
instances = filter_on instances, :realm_id, opts
instances = filter_on instances, :state, opts

You can now use:

instances = filter_on instances, opts, :id, :realm_id, :state

The backward compatibility with old filter_on included.
Mock: Cleanup in MockDriver code

- Removed obsoleted 'requires' (yaml, base64, etc)
- REALMS moved to 'realms' method
- @@METRICS_NAMES moved to Metric model
- Revamped 'filter_on' methods
- Next 'id' generation moved to separate method

a lot of small changes...