[autotest] Make find_hosts_for_jobs a generator

BUG=chromium:672348
TEST=None

Change-Id: I1e764c41c21dd366cbf5fd89f09a935ed6749498
Reviewed-on: https://chromium-review.googlesource.com/560640
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/scheduler/host_scheduler.py b/scheduler/host_scheduler.py
index c4059b7..1fcf896 100755
--- a/scheduler/host_scheduler.py
+++ b/scheduler/host_scheduler.py
@@ -275,15 +275,13 @@
 
         @param host_jobs: A list of queue entries that either require hosts,
             or require host assignment validation through the rdb.
-        @return: A list of tuples of the form (host, queue_entry) for each
+        @return: A generator of tuples of the form (host, queue_entry) for each
             valid host-queue_entry assignment.
         """
-        jobs_with_hosts = []
         hosts = self.acquire_hosts(host_jobs)
         for host, job in zip(hosts, host_jobs):
             if host:
-                jobs_with_hosts.append(self.host_assignment(host, job))
-        return jobs_with_hosts
+                yield self.host_assignment(host, job)
 
 
     def tick(self):