Fix instance_get_all_by_host to actually use slave

This was not fully plumbed out in the original commit,
thus any calls to this method were not actually making
it to a slave.

Original commit: 62e9829c19e099e3a10b2b54954163b5e780363b

Change-Id: If5b807ff9b6801e6038501220e7b6a42bb6132dd
Merge "Fix instance_get_all_by_host to actually use slave"