<oo-accept-node> fix context check for systemctl
<oo-accept-node> clarify uid errmsg when non-gear user is in the range

The error message was for processes owned by UIDs in the gear UID range.
However it stated the UID belonged to a gear no longer on the system. In
my case, they actually belonged to a non-gear user with UID 1000.
Technically that's a problem of sorts (could cause problem w/ districts)
so I still want the error, but tried to make it clearer.
<oo-diagnostics> more specific test conditions

Introduced variables to indicate using the SCL or systemctl,
instead of assuming things based on OS and project.
Makes oo-diagnostics better for Origin and non-RHEL EL.
<oo-diagnostics> more reliable test_broker_certificate
Merge pull request #4344 from sosiouxme/oo-diagnostics

Merged by openshift-bot