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/config.py b/manila_tempest_tests/config.py
index 35fa1ed..c6b9c73 100644
--- a/manila_tempest_tests/config.py
+++ b/manila_tempest_tests/config.py
@@ -30,7 +30,7 @@
                help="The minimum api microversion is configured to be the "
                     "value of the minimum microversion supported by Manila."),
     cfg.StrOpt("max_api_microversion",
-               default="2.50",
+               default="2.51",
                help="The maximum api microversion is configured to be the "
                     "value of the latest microversion supported by Manila."),
     cfg.StrOpt("region",