Bug 1077664: oo-diagnostics: Fix test_node_mco_log when no log

 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.