optimize get_instances_by_node
optimize get_instances_by_node

We can set host filed in search_opts.
refer to:
https://developer.openstack.org/api-ref/compute/?expanded=list-servers-detail#list-servers

Change-Id: I36b27167d7223f3bf6bb05995210af41ad01fc6d

openstack/watcher,stackforge/watcher,stackforge/watcher,openstack/watcher