Cleanup geoserver docker build

[gdooper]

Some minor changes to the geoserver build process. No functional changes to the resulting docker image.

Three main changes are made:

• removing two of the build stages that are unnecessary and possibly confusing
• unpacking the geomesa accumulo and kafka distribution tar files with docker's ADD command, rather than COPY and RUN tar... commands
• refactoring geodocker-geoserver/Makefile to make it more consistent with geodocker-accumulo-geomesa/Makefile