Configurable Compression Level for Cache Artifacts

Allow changing (or disabling) the compression level of tarball artifacts via pants.ini

Also, require passing compression level in most init/callsites rather than allowing various default args to proliferate.

Default compression level in python gzip appears to be 9, so that is the default on TarballArtifact (replacing True).
The default when reading the config is currently 5, as we suspect that we're CPU rather than IO bound, but this change will also make it easier experiment with different values to find confirm that / find the optimal value.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/38756549
https://travis-ci.org/pantsbuild/pants/builds/38964649

Reviewed at https://rbcommons.com/s/twitter/r/1194/
Update config migration script with new scalac plugin options.

Reviewed at https://rbcommons.com/s/twitter/r/1218/