[AIM] External segment implementation

This change adds support for external-segment
and external-policy to the AIM-based GBP driver.
External-segments are implemented through Neutron
external networks (only explicit mode supported).
Association between L3-policy and external-segment
is handled by connecting a router between the
external-network and subnets within the L3policy.
External-policy policy-rule-set relationships also
translate into specific extension attributes of
these routers.

Change-Id: I3417bdadbd65e047fa2b1cf2bcaeedf5fa12503f
Signed-off-by: Amit Bose <amitbose@gmail.com>

gbpservice/neutron/services/grouppolicy/drivers/cisco/apic/apic_mapping_lib.py

@@ -39,6 +39,26 @@ class HierarchicalContractsNotSupported(gpexc.GroupPolicyBadRequest):
     message = _("Hierarchical contracts not supported by APIC driver.")
 
 
+class MultipleExternalPoliciesForL3Policy(gpexc.GroupPolicyBadRequest):
+    message = _("Potential association of multiple external policies to "
+                "an L3 Policy.")
+
+
+class SharedExternalPolicyUnsupported(gpexc.GroupPolicyBadRequest):
+    message = _("APIC mapping driver does not support sharing of "
+                "external policies.")
+
+
+class OnlyOneL3PolicyIsAllowedPerExternalSegment(gpexc.GroupPolicyBadRequest):
+    message = _("Only one L3 Policy per ES is supported when NAT is disabled "
+                "on the ES.")
+
+
+class OnlyOneAddressIsAllowedPerExternalSegment(gpexc.GroupPolicyBadRequest):
+    message = _("Only one ip address on each ES is supported on "
+                "APIC GBP driver.")
+
+
 def get_filter_entries_for_policy_rule(context):
     # forward_rules and reverse_rules is each a dict of filter_entries
     # with each entry in the dict having the filter_entry name as the