Add 'allocation_pools' to Quantum v2 API subnets.

This changeset adds a new parameter, 'allocation_pools' to the 'subnet'
resource of the Quantum v2 API.
Allows for creating and validation of subnets with multiple allocation pools.
This commit only deals with POST verb (subnet creation).
PUT verb (subnet update) will be dealt with in a subsequent commit.

This is the first commit for fixing bug #1016308

Change-Id: Ic356dcb5dcfa46af8ecc7e598529881f8bcbbeed

quantum/common/exceptions.py

@@ -127,6 +127,20 @@ class InvalidContentType(Invalid):
     message = _("Invalid content type %(content_type)s.")
 
 
+class InvalidAllocationPool(QuantumException):
+    message = _("The allocation pool %(pool)s is not valid.")
+
+
+class OverlappingAllocationPools(QuantumException):
+    message = _("Found overlapping allocation pools:"
+                "%(pool_1)s %(pool_2)s for subnet %(subnet_cidr)s.")
+
+
+class OutOfBoundsAllocationPool(QuantumException):
+    message = _("The allocation pool %(pool)s spans "
+                "beyond the subnet cidr %(subnet_cidr)s.")
+
+
 class NotImplementedError(Error):
     pass