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.
Adding Contract resource.

CRUD operations for policy_rule propagated all the way to the
plugin along with UTs.
Adding relationship of EPG with Contracts

This allows creating provider and consumer relationships from
EPG to Contracts.

Note that Selectors are not implemented yet.
create networks for bridge domains
start mapping EP and EPG
fix pep8 issue
use correct Context class
initial mapping of EPGs to subnets
rename backrefs
work on subnet allocation
use polymorphic sqlalchemy relationships
fix plurality of backref names
check cidr availability in RD
map EP to port
implicit BD and RD creation
Merge pull request #2 from noironetworks/rkukura/mapping

initial mapping of RD, BD, EPG, and EP to neutron resources
add mapping items to group_policy_conf.ini
map RD to router
Merge pull request #3 from noironetworks/rkukura/mapping

add config file items, map RD to router
Merge pull request #5 from noironetworks/s3wong/mapping

Add stubs for policy-rule, policy-classifier, and policy-action to mappi...