Adapt to changes in Gerrit's Response handling

Since commit b446055fc03927474885d2eb33c44115873411ef Gerrit core
expected REST endpoints to return a Response Object. This was not yet
the case for the serviceuser plugin, which thus failed the build.

Change-Id: Ifd782cb3ef1f690f8568359b0fb62d4455135beb
16 files changed