Allow tommy_ci to use Jenkins multi-branch projects

Summary:
rTOMMY8d7e01d292da introduced the support for multi-branch projects by default,
while rTOMMYa95280823e1b put this feature behind a guard, requiring to set
the JENKINS_MULTI_BRANCH environment variable.

For tommy_ci, we actually want this so when staging area is enabled,
we could use branches instead of or in complement to the Phabricator plugin.

For tommy_cd, only the main branch is considered, so we don't need it.

Ref T1441.

Test Plan: Browse https://builds.nasqueron.org

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1441

Differential Revision: https://devcentral.nasqueron.org/D2838