Minor adjustments for imagej-omero

Various small fixes and improvements found while working on https://github.com/imagej/imagej-omero/issues?milestone=3&state=open with @ctrueden.

For review:
 * @sbesson et al: thoughts on the addition of `bin/omero admin kill`?
 * Addition of `OMERO_HOME` should have no impact on scripts. (In fact, it can likely be considered part of the public API) /cc @will-moore
 * @ximenesuk: the filtering for `org.bushe` likely isn't needed, but as we will need to consider which logback configuration we're going to pass to background Java tasks.

Happy to break this up if need be, but I wanted to get it open for discussion.