Implement neutron custom constraints for subnet/port/router

Add 3 custom constraints for neutron:
1. 'neutron.subnet' for subnet constraint
2. 'neutron.port' for port constraint
3. 'neutron.router' for router constraint

blueprint neutron-custom-constraint

Change-Id: Iaf8691d59d87ce0e4a55ceacb08223e4ce986608

specs/neutron-custom-constraint.rst

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+==============================================
+ Implement More Custom Constraints for Neutron
+==============================================
+
+https://blueprints.launchpad.net/heat/+spec/neutron-custom-constraint
+
+Now only network constraint is supported for neutron, we need more constraints
+like subnet, port, router etc.
+
+Problem description
+===================
+
+Many resources have some properties related with network, now neutron custom
+constraints only support network constraint, haven't support subnet/port/router
+constraints.
+
+Proposed change
+===============
+
+Add 3 custom constraints to neutron.
+
+1. 'neutron.subnet' for subnet constraint.
+2. 'neutron.port' for port constraint.
+3. 'neutron.router' for router constraint.
+
+Alternatives
+------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  Ethan Lynn
+
+
+Milestones
+----------
+
+Target Milestone for completion:
+  Juno-2
+
+Work Items
+----------
+
+1. Implement subnet constraint for neutron
+2. Implement port constraint for neutron
+3. Implement router constraint for neutron
+
+
+
+Dependencies
+============
+
+https://blueprints.launchpad.net/heat/+spec/glance-parameter-constraint