Use tags for workload selection (#82)

- Remove custom enable-benchmark vars
- Add "default" tag
- Auto-add "all" tag to all benchmarks
- Enable "default" benchmarks
- Remove disabledByDefault checks
- Remove forceEnable mechanism (we can always create a custom tag for debugging)
- Add defaultDisabledTags for WorkerTests in shells
- Support tag as urlParam

Author: camillobruni