Adds new functional tests for share-network-subnets

This patch adds functional test for share-network-subnets and
validates the new API version 2.51

Change-Id: I60bdb493ad5766f28408a0c877f960922fe44ad0
Partially-implements: bp share-network-multiple-subnets
diff --git a/manila_tempest_tests/utils.py b/manila_tempest_tests/utils.py
index c2c7ec6..09effde 100644
--- a/manila_tempest_tests/utils.py
+++ b/manila_tempest_tests/utils.py
@@ -22,6 +22,7 @@
 import testtools
 
 CONF = config.CONF
+SHARE_NETWORK_SUBNETS_MICROVERSION = '2.51'
 
 
 def get_microversion_as_tuple(microversion_str):
@@ -177,3 +178,13 @@
         raise testtools.TestCase.skipException(
             "Share manage tests with multitenancy are disabled for "
             "microversion < 2.49")
+
+
+def share_network_subnets_are_supported():
+    return is_microversion_supported(SHARE_NETWORK_SUBNETS_MICROVERSION)
+
+
+def share_network_get_default_subnet(share_network):
+    return next((
+        subnet for subnet in share_network.get('share_network_subnets', [])
+        if subnet['availability_zone'] is None), None)