Use OVERRIDE_ZUUL_BRANCH in alternate-branch jobs

Requires (and required by) https://review.openstack.org/64158

Change-Id: I606bb8d1dd2ef0a8043a2e84404ad9df18b35ac4
Stop unsetting ZUUL_PROJECT in periodic jobs

Needs https://review.openstack.org/#/c/64162/ .  Once that lands
unsetting ZUUL_PROJECT is no longer required.

Change-Id: Ie9c9ab3c987c5f1dfe7e1752ef595ce5a9f1b8c4
Merge "Use OVERRIDE_ZUUL_BRANCH in alternate-branch jobs"
Merge "Stop unsetting ZUUL_PROJECT in periodic jobs"