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: I985d0c0bf29218cf503948a62e08b7c604a1000d
1 file changed
tree: a78e97562fc64c6020c8c57bd49188693cced931
  1. ceilometer/