[FIXED JENKINS-15869] When running on Java 5, must still load...

[FIXED JENKINS-15869] When running on Java 5, must still load DiskSpaceMonitor/TemporarySpaceMonitor, even though they will show N/A.
Returning null from an @Extension method does not seem to work reliably, and is not supported by SezPoz.
If desirable to hide a monitor column in $server/computer/, add a boolean isEnabled() method interpretable in Jelly.