Check if Openstack connection exists before trying to remove it from connections

Commit 08defa6a5ff7067e89565a49df69d9450394453c assumed that openstack connections will always exists in connections. However
when run just fab setup_vrouter, connections are not available and it fails

Change-Id: I5e31f929c63c9caad6047a551eed48f87cf91c63
Closes-Bug: 1382625