after Failed over 'node_id': ok button failover should be disabled, otherwise we can failover node many times

build 1715

steps, as example:
1.5 nodes in the cluster
2.shutdown 1 node
3.after node is marked as Down, click "Failover"

result:
Starting failing over 'ns_1@10.3.3.15'
Failed over 'ns_1@10.3.3.15': ok

button "Fail over" is enabled and we can Fail Over node many times:

Failed over 'ns_1@10.3.3.15': ok (repeated 6 times) ns_orchestrator006 ns_1@10.3.3.6 14:40:45 - Wed Sep 12, 2012
Starting failing over 'ns_1@10.3.3.15' (repeated 3 times) ns_orchestrator000 ns_1@10.3.3.6 14:40:45 - Wed Sep 12, 2012
Starting failing over 'ns_1@10.3.3.15' ns_orchestrator000 ns_1@10.3.3.6 14:39:55 - Wed Sep 12, 2012
Starting failing over 'ns_1@10.3.3.15' (repeated 2 times) ns_orchestrator000 ns_1@10.3.3.6 14:39:45 - Wed Sep 12, 2012
Failed over 'ns_1@10.3.3.15': ok ns_orchestrator006 ns_1@10.3.3.6 14:39:34 - Wed Sep 12, 2012
Starting failing over 'ns_1@10.3.3.15'

it doesn't make sense and it's better to disable the 'Failover' button after first successful attempt