Add defaults to allow job to run without Zuul

Running the dsvm-tempest-full job directly from Jenkins without Zuul
means some variables needed by devstack-vm-gate.sh and functions.sh
are not set, and this cause the job to timeout and fail.

Some non-blank setting for these is needed or these give errors, and for some reason the job gets stuck and times out for me.

 $ZUUL_PROJECT is needed in functions.sh:
https://github.com/openstack-infra/devstack-gate/blob/a4ef9faee23e5ef0da9abad889252424a358664d/functions.sh#L251

$ZUUL_BRANCH is needed by setup_localrc in devstack-vm-gate.sh:
https://github.com/openstack-infra/devstack-gate/blob/a4ef9faee23e5ef0da9abad889252424a358664d/devstack-vm-gate.sh#L269