Support custom local docker registry for local experiments.

Hi

I started to run a local experiment as described [here](https://google.github.io/fuzzbench/getting-started/prerequisites/) and [here](https://google.github.io/fuzzbench/running-a-local-experiment) .

I run the following command :

`PYTHONPATH=. python3 experiment/run_experiment.py --experiment-config experiment-config.yaml --benchmarks freetype2-2017 bloaty_fuzz_target --experiment-name $EXPERIMENT_NAME --fuzzers afl libfuzzer
`
the `experiment-config.yaml` has the following content :
```
# The number of trials of a fuzzer-benchmark pair.
trials: 5

# The amount of time in seconds that each trial is run for.
# 1 day = 24 * 60 * 60 = 86400
max_total_time: 1800

# The location of your docker registry.
docker_registry: lab-server:5000

# The local experiment folder that will store most of the experiment data.
# Please use an absolute path.
experiment_filestore: /tmp/experiment-data

# The local report folder where HTML reports and summary data will be stored.
# Please use an absolute path.
report_filestore: /tmp/report-data
```

But I got the following error :

```
ERROR:root:Config does not contain "cloud_project".
ERROR:root:Config does not contain "cloud_compute_zone".
ERROR:root:Config parameter "experiment_filestore" is "/tmp/experiment-data". It must start with gs:// when running on Google Cloud.
ERROR:root:Config parameter "report_filestore" is "/tmp/report-data". It must start with gs:// when running on Google Cloud.
Traceback (most recent call last):
File "experiment/run_experiment.py", line 483, in
sys.exit(main())
File "experiment/run_experiment.py", line 478, in main
no_dictionaries=args.no_dictionaries)
File "experiment/run_experiment.py", line 214, in start_experiment
config = read_and_validate_experiment_config(config_filename)
File "experiment/run_experiment.py", line 132, in read_and_validate_experiment_config
raise ValidationError('Config: %s is invalid.' % config_filename)
__main__.ValidationError: Config: experiment-config.yaml is invalid.
```

I did everything as documented but I came up with this .

I did not understand the following in config file :
`docker_registry: lab-server:5000
`

Should I setup a docker_registry ? if yes how to do this ?

thnx