Apply a naming rule of GET to show_server method

[GET /resources] methods should be "list_<resource name>s"
or "show_<resource name>", so this patch applies the rule
to show_server method of compute clients.

Partially implements blueprint consistent-service-method-names

Change-Id: I92addd2ad6502fe00395e2b9455de25c2f53d115
diff --git a/tempest/api/compute/admin/test_aggregates.py b/tempest/api/compute/admin/test_aggregates.py
index 35a6479..f103cb6 100644
--- a/tempest/api/compute/admin/test_aggregates.py
+++ b/tempest/api/compute/admin/test_aggregates.py
@@ -211,5 +211,5 @@
         server = self.create_test_server(name=server_name,
                                          availability_zone=az_name,
                                          wait_until='ACTIVE')
-        body = admin_servers_client.get_server(server['id'])
+        body = admin_servers_client.show_server(server['id'])
         self.assertEqual(self.host, body[self._host_key])