Add ovirt-engine-4.0.6 branch to the list of stable branches

Otherwise gerrit-hooks ignores it: (taken from gerrit.ovirt.org/67324) gerrit-hooks Nov 25 19:19 ↩ Patch Set 1: - Update Tracker::#1398550::IGNORE, not relevant for Red Hat classification - Check Bug-Url::IGNORE, not relevant for 'Red Hat' classification - Check Public Bug::#1398550::OK, public bug - Check Product::#1398550::IGNORE, not relevant for classification: Red Hat - Check TM::#1398550::IGNORE, not relevant for classification: Red Hat - Set POST::#1398550::IGNORE, not relevant for Red Hat classification - Check merged to previous::IGNORE, Not in stable branch (['ovirt-engine-3.6', 'ovirt-engine-4.0', 'ovirt-engine-3.6.9', 'ovirt-engine-4.0.2', 'ovirt-engine-4.0.4'])