Improve bgp_stress_test teardown code
Improve bgp_stress_test teardown code

If the XmppServer is shutdown before the agents, the agents keep
trying to connect to it continuously and keep the scheduler busy.
Instead, cleanup the agents first and then shutdown XmppServer.

Change-Id: Iea6a65c907f4dddbc5f42c787e4d3974ffd6f3b2
Partial-Bug: 1464016

sajuptpm/contrail-controller,codilime/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,DreamLab/contrail-controller,vpramo/contrail-controller,nischalsheth/contrail-controller,vmahuli/contrail-controller,facetothefate/contrail-controller,hthompson6/contrail-controller,reiaaoyama/contrail-controller,DreamLab/contrail-controller,hthompson6/contrail-controller,tcpcloud/contrail-controller,reiaaoyama/contrail-controller,eonpatapon/contrail-controller,tcpcloud/contrail-controller,sajuptpm/contrail-controller,codilime/contrail-controller,codilime/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,vpramo/contrail-controller,tcpcloud/contrail-controller,vmahuli/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,vpramo/contrail-controller,eonpatapon/contrail-controller,reiaaoyama/contrail-controller,numansiddique/contrail-controller,facetothefate/contrail-controller,vmahuli/contrail-controller,nischalsheth/contrail-controller,DreamLab/contrail-controller,vpramo/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,numansiddique/contrail-controller,facetothefate/contrail-controller,nischalsheth/contrail-controller,sajuptpm/contrail-controller,DreamLab/contrail-controller,sajuptpm/contrail-controller,sajuptpm/contrail-controller,hthompson6/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,numansiddique/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,vmahuli/contrail-controller,numansiddique/contrail-controller,reiaaoyama/contrail-controller,rombie/contrail-controller,sajuptpm/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,tcpcloud/contrail-controller,codilime/contrail-controller,numansiddique/contrail-controller,DreamLab/contrail-controller,facetothefate/contrail-controller,vmahuli/contrail-controller,eonpatapon/contrail-controller,hthompson6/contrail-controller,reiaaoyama/contrail-controller,hthompson6/contrail-controller,rombie/contrail-controller,codilime/contrail-controller,vpramo/contrail-controller,facetothefate/contrail-controller