mergify: add configuration for 4.2z1 branch
mergify: add configuration for 4.2z1 branch

So we get backports against 4.2z1 branch (downstream related) automatically
created by mergify

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible