oo-diagnostics: Drop unneeded require 'socket'

Drop a "require 'socket'" statement in the test_broker_certificate check.

This require statement was obviated in commit
9b72939f28a6d35354db74cc7464d879bbd3a81e.
oo-diagnostics: Better handle curl error

Catch and report if curl returns an error in the test_broker_certificate
check.

Previously, test_broker_certificate would ignore curl's return code, and so
errors connecting to the broker would turn up as backtraces when
test_broker_certificate would try to parse the output from curl.
Merge pull request #5324 from Miciah/oo-diagnostics-test_broker_certificate-fixes

Merged by openshift-bot