Define minimum required API ver for portgroups

This patch defines min_microversion for API portgroup tests to
skip them when this API version is not supported/available.
For example on stable/mitaka branches.

Change-Id: I8895e1e6a819b5b3821fdfc10e1cff8626c9eb29
diff --git a/ironic_tempest_plugin/tests/api/admin/test_portgroups.py b/ironic_tempest_plugin/tests/api/admin/test_portgroups.py
index ceb1d11..25e1133 100644
--- a/ironic_tempest_plugin/tests/api/admin/test_portgroups.py
+++ b/ironic_tempest_plugin/tests/api/admin/test_portgroups.py
@@ -21,10 +21,13 @@
 class TestPortGroups(base.BaseBaremetalTest):
     """Basic positive test cases for port groups."""
 
+    min_microversion = '1.23'
+
     def setUp(self):
         super(TestPortGroups, self).setUp()
         self.useFixture(
-            api_microversion_fixture.APIMicroversionFixture('1.25'))
+            api_microversion_fixture.APIMicroversionFixture(
+                self.min_microversion))
         _, self.chassis = self.create_chassis()
         _, self.node = self.create_node(self.chassis['uuid'])
         _, self.portgroup = self.create_portgroup(