oo-diagnostics: Add test_node_clock_in_synch_with_broker

Add a `test_node_clock_in_synch_with_broker` check to `oo-diagnostics`.  This test is specific to node hosts.  It sends an HTTP request to the host identified by the `BROKER_HOST` setting in `/etc/openshift/node.conf` and compares the date in the Date: header of the response to the date of the node host.  If the difference is equal to or more than 55 seconds, the check gives an error.  Else, if the difference is equal to or more than 5 seconds, the check gives a warning.

This commit fixes bug 1121267.