Revert "create jenkins user with systemuser"

Come back to using the user {} definition. systemuser {} defines the
group which would prevents us from keeping the group definition inside
the jenkins module.

This reverts commit bff1865e793de6aa812fe18cfd5b913210e8588e.

Change-Id: I7482ca5dce574e6557656b8c15d640b07df20b0c