Use generic user for both zuul v2 and v3
Use generic user for both zuul v2 and v3

Zuul v2 uses 'jenkins' as user, but Zuul v3 uses 'zuul'.
Using $USER solves it for both cases.

Change-Id: I8cddbefaf006b6d54285fffe2a245a4a8ea8b251

openstack/devstack-plugin-ceph,ajarr/devstack-plugin-ceph