Allow namespaces to be removable.

When network namespaces are used for dhcp and l3, we need to allow
router_delete_namespaces and dhcp_delete_namespaces to be overridden so
that the namespaces can be deleted.

Change-Id: I8255f0a43ffd1100f971470208103503e9dd8229
Closes-Bug: #1336613
Merge "Allow namespaces to be removable."