Record build_branch in logstash

Since some bugs are branch specific we want to write logstash queries
that use the branch, so log ZUUL_BRANCH as build_branch in logstash.

From zuul's launchers doc:
ZUUL_BRANCH: The target branch for the change that triggered this build

Change-Id: Ic408afb235be5716231c663616c17a98ef6f8870
Merge "Record build_branch in logstash"