Add ability to use an existing local Notes or Domino installation for the platform

[jesse-gallagher]

Currently, local builds require a notes-platform property pointing to a generated p2 site, such as via generate-domino-update-site. However, it should be entirely doable to be able to use the notes-program property (or another property) to locate the osgi directory for a local installation and use that instead, simplifying setup a bit.