Pool aware multi-backend filter

In a multi backend scenario a user should be able to specify the backend providers to be used when creating a volume. The current multi backend support is restricted to grouping backends in the conf file and is not pool aware.

This proposal is to allow users to specify a list of backend providers along with the pool information in the volume_type for the scheduler to pick from. This helps grouping the backends#pool to better suit a user requirement. Say user_a is allowed to create volumes from provider1, provider2 and user_b from provider2,provider3, also all provider1 volumes to be used by user_a should come from pool1. The volumes to be used by user_a could be of type user_a_type which has a property ["provider1#pool1", "provider2"]