tf-a-builder: add GERRIT_HOST parameter

Without GERRIT_HOST SQUAD is not able to send review comments/scores to
proper gerrit server. This patch passes GERRIT_HOST from trigger builds
down to tf-a-builder.

Signed-off-by: Milosz Wasilewski <milosz.wasilewski@linaro.org>
Change-Id: Ib28cd136a01059d36c677b2c4019b993a1aba90d
5 files changed