oo-diagnostics: Fix test_node_mco_log when no log

Make test_node_mco_log fail gracefully when the node mcollective log file
is absent.  Before this commit, the test would hang because it would run
a sed command with no input file provided.

This commit fixes bug 1077664.
oo-diagnostics: Fix comments for test_node_mco_log

Place comments with relevant code, consistently use proper case and
punctuation, and use whitespace for better logical grouping.
Merge pull request #4983 from Miciah/bug-1077664-oo-diagnostics-test_node_mco_log-fixes

Merged by openshift-bot