verify: --clean-up-verified to rm all files except the JSON report

The verification server can easily fill up with APKs and src tarballs. But they are only useful if the verification failed, e.g. for troubleshooting. This adds a flag to clean up all the files except the JSON report if the verification was successful.

@uniqx I thought this could be merged as a standalone, so I opened this. If you think it would be better to keep it in the buildbot-subcommands dev branch, we can do that.