scripts: test-bifrost.sh: Initialize the ZUUL_BRANCH variable
scripts: test-bifrost.sh: Initialize the ZUUL_BRANCH variable

Commit afa9951dd92e2 ("Move database setup to bifrost") moved the
openstack_citest database setup from project-config to bifrost.
However, we need to provide a default (even if it's an empty one)
value for ZUUL_BRANCH (eg when running outside of zuul) in order
to not fail due to using 'set -u'. Fixes the following error:

./scripts/test-bifrost.sh: line 76: ZUUL_BRANCH: unbound variable

Change-Id: Iea1193ce0ac83013f4e41398b4e7ad9527d427ac
Fixes: afa9951dd92e2 ("Move database setup to bifrost")

openstack/bifrost,openstack/bifrost