nextstrain-automation: Include rules via custom_rules

[joverlee521]

Description of proposed changes

Using custom_rules allows the nextstrain-automation build to skip the config validation, which fails because of the extra deploy_url param.

Related issue(s)

Follow up to #139

Checklist

• Checks pass
• Update changelog Internal changes

[joverlee521]

Merging since dry-run shows the expected output where schema validation is skipped

$ nextstrain build . deploy -n --configfile config/nextstrain_automation.yaml -q
Using workflow specific profile profiles/default for setting default command line arguments.
Config file config/configfile.yaml is extended by additional config specified via the command line.
Saved 'config' to 'results/run_config.yaml'.
WARNING: Skipping config schema validation because custom rules are defined.
host: 8123212ce4fd
Building DAG of jobs...