Remove "Delete" buttons when VPN objects are in use

Deleting attached services and policies doesn't
cause NotImplementedError anymore. Moreover,
options "Delete VPN Service", "Delete IKE Policy" and
"Delete IPSec Policy" don't appear in corresponding row
if there is a connection created using these objects.
Some tests are added to detect such errors in future.

Fixes bug 1252330

Change-Id: I9ed1b4ecdad54b89bef45b2f5e422c30447cceb5
Merge "Remove "Delete" buttons when VPN objects are in use"