Allow overwritting xwiki.properties and xwiki.cfg properties via environment variable and Java system properties

The idea is to make it easier for use cases which cannot modify the filesystem (or for which it's very complex) to provide configuration customization.

See https://forum.xwiki.org/t/add-a-generic-way-to-overwrite-some-configuration-through-environment-variables-and-java-system-properties/16711/5 for more details