Fixes HostTestCase failures due to slow test run

Status check in _list_hosts() fails if tests take longer than the
service_down_time.

Stubbing utcnow() guarantees that status check passes for tests.

Change-Id: I188443730ce077a8117812f1089db36ece184e7a
Closes-Bug: 1292643
(cherry picked from commit 4465cd20b42c0db0efd405b57d9dc141bf6ed918)