provision/juju: fix elbInstanceHealer

Now it won't enter in an infinite loop healing instances :-)
provision/juju: make test more reliable

String sorting, i-10 < i-9 :-)
provision/juju: change name of field

instance.instanceId is just verbose. Let's not write Java code :-)
provision/juju: added cache in elbInstanceHealer

Just a premature optmization :-)