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

[gdooper]

I'm not sure if it's better to download this geomesa-accumulo and geomesa-kafka distributions from a publicly available location (e.g. github releases), or from a local build.
@elahrvivaz do you have a preference?

[elahrvivaz]

can we check for a local version, and download if it doesn't exist?

[elahrvivaz]

actually, since we cache it locally then we can just download it, but check to see if it's cached already first