There exist a case where deleting router from neutron without
deleting router-interface deletes router from VSD but not from neutron.
If router has an interface with subnet router deletion is not allowed.
Needs to precheck this condition before deleting router from
backend (VSD).
Change-Id: I4ba7eff399acda0427505328355dbfb931d62587
Closes-Bug: #1298635
10ee13496f