FTR: add 'throttle' option to cli

[dmlemeshko]

Summary

Run tests with enabled network throttling or headless browser from cli:

kibana git:(ftr-cli-throttling-option) ✗ node scripts/functional_test_runner --throttle
debg Loading config file from "/Users/dzmitrylemechko/github/kibana/test/functional/config.js"
debg Loading config file from "/Users/dzmitrylemechko/github/kibana/test/common/config.js"
info Config loaded
debg randomness seed: 1575903801698
debg [webdriver] Creating session
**debg NETWORK THROTTLED: 768k down, 256k up, 100ms latency.**
info Remote initialized: chrome 78.0.3904.108

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

- [ ] This was checked for cross-browser compatibility, including a check against IE11
- [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support

• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
  - [ ] This was checked for keyboard-only and screenreader accessibility

For maintainers

- [ ] This was checked for breaking API changes and was labeled appropriately
- [ ] This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[dmlemeshko]

@elasticmachine merge upstream

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 35c630a

History

• 💚 Build #14606 succeeded 1c26fdc
• 💔 Build #14236 failed 475dad8

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[spalger]

LGTM

[tylersmalley]

@dmlemeshko is there a reason we didn't backport this to the 7.x branch?