Allow non-critical updates to be triggered from the command line

Commit 2494e28b ("Skip non-critical updates during OOBE update check")
added a new requirement: if the omaha response did not specify a deadline,
the update would not be applied until OOBE finished.  But the lab tests
and provisioning processes often do require updates to be applied before
OOBE finishes.  So we will exempt updates triggered via
`update_engine_client --update` from this new requirement, while keeping
the requirement for update checks triggered from Chrome.

Bug: None
BUG=chromium:587101
BUG=chromium:600737
TEST=manually initiate updates from the OOBE flow and from
     update_engine_client, and watch /var/log/update_engine.log for
     results
TEST=`FEATURES=test emerge-cyan chromeos-base/update_engine`

Change-Id: I7671b92d0d0dbb871e8f2c737c01e91cba29fa9d