Add support for setting system properties via user supplied .properties files

Many system options and features are controlled via system properties, which are found in the following files:

• bootstrap.properties
• rrd-configuration.properties
• libraries.properties
• opennms.properties

Additionally, system properties may be set via the Java command line in opennms.conf

As these files change between releases, mostly rrd-configuration.properties and opennms.properties, users are forced to merge their configuration files during the upgrade process.

In order to make upgrading, and configuration management easier, I propose the following:

• Keep the existing property files in place with the recommended defaults
• Allow users to override the properties set by the distributed via their own property files