NMS-6644: Initial revision of the DiskUsageMonitor

http://issues.opennms.org/browse/NMS-6644
NMS-6644: Some correction requested after first review by Ronny
NMS-6644: Fixed another typo
Merge branch 'cyr-doc-DiskUsageMonitor' of https://github.com/StCyr/opennms into StCyr-cyr-doc-DiskUsageMonitor

Conflicts:
	opennms-doc/guide-admin/src/asciidoc/index.adoc
Merge branch 'StCyr-cyr-doc-DiskUsageMonitor' into upstream-dev-docs