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
diff --git a/tf-a-builder.yaml b/tf-a-builder.yaml
index 9875e09..84ab556 100644
--- a/tf-a-builder.yaml
+++ b/tf-a-builder.yaml
@@ -104,6 +104,9 @@
             name: GERRIT_CHANGE_NUMBER
             default: ''
         - string:
+            name: GERRIT_HOST
+            default: 'review.trustedfirmware.org'
+        - string:
             name: QA_SERVER_TEAM
             default: 'tf'
         - string: