[AIM NFV] Limit the number of PPG per PC to 3

PPG: Port Pair Group
PC: Port Chain

We limit the number of services in a chain to three
as suggested by the ACI guidelines

Change-Id: Ibf6cc78a1f0363eb45767a594318e28edd0235ab

gbpservice/neutron/services/sfc/aim/exceptions.py

@@ -112,3 +112,7 @@ class ConflictingNetworksDetectedInPortChain(exceptions.BadRequest):
 class DefaultExternalNetworkNotFound(exceptions.NotFound):
     message = _("Default External Network not found for SVI network "
                 "%(id)s.")
+
+
+class TooManyPPGsPerChainError(exceptions.BadRequest):
+    message = _("The max number of PPGs per chain supported is %(maxn)s.")

gbpservice/neutron/services/sfc/aim/sfc_driver.py

@@ -42,6 +42,7 @@ FLOWC_DST = 'dst'
 LOG = logging.getLogger(__name__)
 PHYSDOM_TYPE = 'PhysDom'
 SUPPORTED_DOM_TYPES = [PHYSDOM_TYPE]
+MAX_PPGS_PER_CHAIN = 3
 
 
 class SfcAIMDriverBase(base.SfcDriverBase):
@@ -290,6 +291,8 @@ class SfcAIMDriver(SfcAIMDriverBase):
         #   can be removed once contract export is implemented.
         # TODO(ivar): two different chains cannot share left/right networks
         # TODO(ivar): right/left BDs same tenant as provider
+        if len(ppgs) > MAX_PPGS_PER_CHAIN:
+            raise exceptions.TooManyPPGsPerChainError(maxn=MAX_PPGS_PER_CHAIN)
         vrfs = set()
         for flowc in flowcs:
             provg = self._get_flowc_provider_group(p_ctx, flowc)