Make test_server work with older versions of oslo.service

Make test_server work with older versions of oslo.service

Change I18a11283925369bc918002477774f196010a1bc3 fixed the test for
oslo.service >= 0.10.0, but it also broke it for older versions of
oslo.service. Since the library has minimal version of >= 0.7.0 in
requirements.txt, test should pass for those versions too.

Now, instead of validating that either reset() or restart() of workers
are triggered on SIGHUP, just validate that .start() is triggered the
expected number of times (either way, no matter how oslo.service decide
to clean up the children, they exit and then are respawned).

Change-Id: I41f9d3af780b3178b075bc1e7084f417a2bd1378
Closes-Bug: #1505645