List only alive agents
List only alive agents

When check the HA agent list for router rescheduling,
we should list only the alive agent list.

Change-Id: I4b419697345496ec0027e7d5a8dca690e2351e82
Closes-Bug: #1572442

masayukig/tempest,vedujoshi/tempest,Juniper/tempest,openstack/tempest,sebrandon1/tempest,vedujoshi/tempest,Juniper/tempest,Tesora/tesora-tempest,openstack/tempest,masayukig/tempest,bigswitch/tempest,bigswitch/tempest,cisco-openstack/tempest,sebrandon1/tempest,cisco-openstack/tempest,Tesora/tesora-tempest