Support the value_specs property in all Neutron resources

Neutron resources already support the value_specs property for a few resources like subnet, port, firewall, floating IP, router, and network. This property can be used to pass (in a dictionary fashion) other parameters than the pre-defined ones when creating an instance of such a resource. This property is useful when extending Neutron and Heat as custom parameters can be passed using value_specs.

The goal of this blueprint is to bring the value_specs property to all Neutron resources in Heat.