Changing the way EPGs consume and provide contracts.

The consumed_contract_scopes and provided_contract_scopes
attributes in the EPG were earlier lists containing
ContractProvidingScope or ContractConsumingScope uuids.
Changing these to be a dict such that

Key = Contract ID
Value = ID of ContractProvidingScope or ContractConsumingScope
        or default is None

Note that ContractProvidingScope and ContractConsumingScope
no longer have the Contract ID attribute.

Doing it this way makes it optional to use ContractProvidingScope
or ContractConsumingScope.