Increase address allocation size for subnet pool tests with IPv4

This patch is increasing the address allocation size for the subnet pool
tests with IPv4 (SubnetPoolsTest class) by:

* Changing the subnet_cidr from /31 to /29
* Changing the max_prefixlen from 31 to 30

The reason for that is because some ML2 drivers such as OVN would create
a new port in Neutron at the moment that a network is created for being
used by the metadata agent. This new port will allocate an IP from the
subnet which was causing the tests below to fail with a "No more IP
addresses available on network ..." error:

SubnetPoolsTest.test_create_subnet_from_pool_with_prefixlen
SubnetPoolsTest.test_create_subnet_from_pool_with_quota
SubnetPoolsTest.test_create_subnet_from_pool_with_subnet_cidr

For more information see:
http://lists.openstack.org/pipermail/openstack-dev/2018-April/129084.html

Related-Bug: #1761119
Change-Id: If5098f6e60742421022712231589afe6755878dc
diff --git a/neutron_tempest_plugin/api/test_subnetpools.py b/neutron_tempest_plugin/api/test_subnetpools.py
index dc822e3..ec3753a 100644
--- a/neutron_tempest_plugin/api/test_subnetpools.py
+++ b/neutron_tempest_plugin/api/test_subnetpools.py
@@ -52,9 +52,9 @@
 class SubnetPoolsTest(SubnetPoolsTestBase):
 
     min_prefixlen = '28'
-    max_prefixlen = '31'
+    max_prefixlen = '30'
     _ip_version = 4
-    subnet_cidr = u'10.11.12.0/31'
+    subnet_cidr = u'10.11.12.0/29'
     new_prefix = u'10.11.15.0/24'
     larger_prefix = u'10.11.0.0/16'