Fix Manual Triggering of Docker Jobs

Fix Manual Triggering of Docker Jobs

When manually triggering a docker job (or even through timed builds),
GERRIT_EVENT_TYPE will not get set. This does gets set when the job is
triggered by a Gerrit event (ex: ref-updated, comment-added).

Specifying the variable lookup to default to nothing (ex: "${FOO:-}")
should allow non-Gerrit triggered jobs to complete successfully.

Change-Id: Ic1ba8ebef0967b0f20a2d6385ce0e0b4876901dc
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>