Rename get_server_group to show_server_group

[GET /resources] methods should be "list_<resource name>s"
or "show_<resource name>", so this patch applies the rule
to server_group client.

Partially implements blueprint consistent-service-method-names

Change-Id: I75307176c2f032301da6227249070c42af54dd6e
diff --git a/tempest/api/compute/servers/test_server_group.py b/tempest/api/compute/servers/test_server_group.py
index 0da7912..9a04556 100644
--- a/tempest/api/compute/servers/test_server_group.py
+++ b/tempest/api/compute/servers/test_server_group.py
@@ -104,9 +104,9 @@
             self._delete_server_group(server_groups[i])
 
     @test.idempotent_id('b3545034-dd78-48f0-bdc2-a4adfa6d0ead')
-    def test_get_server_group(self):
+    def test_show_server_group(self):
         # Get the server-group
-        body = self.client.get_server_group(
+        body = self.client.show_server_group(
             self.created_server_group['id'])['server_group']
         self.assertEqual(self.created_server_group, body)