publish build logs with rsync

Implemented log file upload after each individual build run as described in #495 (closed). Plus some basic regression tests.

After each individual build the corresponding log will get uploaded to the configured serverwebroot via rsync. I did not add any of the commandline ssh-config arguments available for fdroid server to fdroid build. (--identity-file, --no-checksum) That seemed out of place. Having identity_file configured in config.py is supported, but I'd recommend use .ssh/config for doing ssh configuration.

Publishing logs this way is only supported when builds are running on a buildserver VM