Enable java properties overlays in metacat

[artntek]

As a metacat deployer, I need a way of defining a subset of properties in a smaller /var/metacat/metacat-site.properties file, which will be used at runtime to override the default values in WEB-INF/metacat.properties.

However, note that there is an existing "properties backup" scheme whereby metacat backs up changed values in /var/metacat/.metacat/ so they are not lost during an upgrade. It would ultimately be preferable for those changed values to be written to this new /var/metacat/metacat-site.properties file and overlayed as described above, although this is not a high priority for now. At a minimum, these new changes must be able to coexist with and not break the old "backups" method.
⬆️ this has been moved to issue #1628 -- see below

Issues related to this epic:

• Replace Old "Backup Properties" scheme with new "Site Properties" Approach #1628
• Add new/updated documentation explaining how the properties overlay system works #1630
• Change edu.ucsb.nceas.metacat.oaipmh code so it uses PropertyService instead of manipulating metacat.properties directly #1632

[artntek]

Started a google doc to give an overview of the new properties overlay functionality, and to collect the different installation/upgrade scenarios we need to support, so we can ensure the new approach covers these properly.

[artntek]

TODO: Update documentation

moved to new Issue #1630
done