Tests proceeding before fabric xconnect removed

At a number of points, the SiaB tests need to wait for the fabric crossconnect to be torn down before proceeding.  Currently they do this by waiting for the Fabric Crossconnect ServiceInstance to be deleted.  However, there is a delay between when this object is deleted and when the xconnect is removed in ONOS.  This introduces a race condition that causes tests to occasionally fail (e.g., DHCP still works when it is expected not to).

A straightforward fix would be to wait until the xconnect is removed in ONOS.  For instance something like this:

http -a karaf:karaf GET http://127.0.0.1:30120/onos/segmentrouting/xconnect|jq -r '.xconnects[].vlanId'|grep 222