HUE-1256 [core] Cache the HA jt

MR1 HA works by specifying multiple MR clusters in hue.ini.
Default cluster is default.
If default disappear, the next RUNNING JT is picked and cached.
This is done by calling cluster.get_next_ha_mrcluster().
This is done transparently when navigating in JobBrowser or
submitting an Oozie job.