Default subnet-prefix-length should be consistent

Changed default_subnet_prefix_length from "default=26"
to "default=24" in implicit_policy.ini and implicit_policy.py
for consistency.

Updated relevant unit tests.

Closes-Bug: #1489981

Change-Id: I2356191b4de2e182e27a6070791e3c57a6db80c8
(cherry picked from commit a6c5f78630)

etc/drivers/implicit_policy.ini

@@ -19,7 +19,7 @@
 # polices, controlling size of subnets allocated for
 # policy target groups.
 
-# default_subnet_prefix_length = 26
+# default_subnet_prefix_length = 24
 
 # Name of default External Segment. This will be used
 # whenever a new EP/L3P is created without a referenced

gbpservice/neutron/services/grouppolicy/drivers/implicit_policy.py

@@ -37,7 +37,7 @@ opts = [
                       "from which subnets are allocated for policy target "
                       "groups.")),
     cfg.IntOpt('default_subnet_prefix_length',
-               default=26,
+               default=24,
                help=_("Subnet prefix length for implicitly created default L3 "
                       "polices, controlling size of subnets allocated for "
                       "policy target groups.")),

gbpservice/neutron/tests/unit/db/grouppolicy/test_group_policy_db.py

@@ -502,7 +502,7 @@ class TestGroupResources(GroupPolicyDbTestCase):
         ctx = context.get_admin_context()
         data = {'l3_policy': {'name': 'l3p1', 'ip_version': 4,
                               'description': '', 'ip_pool': '0.0.0.0/0',
-                              'subnet_prefix_length': 26}}
+                              'subnet_prefix_length': 24}}
 
         self.assertRaises(gpolicy.InvalidIpPoolPrefixLength,
                           self.plugin.create_l3_policy, ctx, data)
@@ -533,7 +533,7 @@ class TestGroupResources(GroupPolicyDbTestCase):
     def test_update_l3_policy(self):
         name = "new_l3_policy"
         description = 'new desc'
-        prefix_length = 26
+        prefix_length = 24
         es = self.create_external_segment()['external_segment']
         es_dict = {es['id']: ['172.16.0.2', '172.16.0.3']}
         attrs = cm.get_create_l3_policy_default_attrs(

gbpservice/neutron/tests/unit/services/grouppolicy/test_implicit_policy.py

@@ -190,6 +190,7 @@ class TestImplicitL3Policy(ImplicitPolicyTestCase):
         l3p = self.deserialize(self.fmt, req.get_response(self.ext_api))
         self.assertEqual('default', l3p['l3_policy']['name'])
         self.assertEqual(shared, l3p['l3_policy']['shared'])
+        self.assertEqual(24, l3p['l3_policy']['subnet_prefix_length'])
 
         # Create 2nd L2 policy sharing implicit L3 policy.
         l2p2 = self.create_l2_policy()