Inject gerrit-doc.properties for gate-{name}-tox-doc-publish-{envlist}

Our publishing jobs needs gerrit-doc.properties to figure out on which
branch they run. This was removed by accident when moving from maven to
tox jobs. The scripts work fine for master branch without the file but
fail on the stable branch.

Change-Id: I1acb3f6de3a449da6fad23bab1568ba9db7d2c47
Merge "Inject gerrit-doc.properties for gate-{name}-tox-doc-publish-{envlist}"