Also fix JSON schema, that was just invalid.

Now we have a schema that renders properly on
https://lab.nexedi.com/bk/rjs_json_form and is similar to the one from
erp5, so maybe one day we can have a convention on erp5 testnode to push
the URL of a selenium server as test-runner.server-url . In any case,
it's good for now to have consistency.

Most of this work comes from Boris Kocherov from https://www.raskon.org/
Thanks !

This is a backward incompatible parameter change.

for example to run on saucelabs, instead of:
  {
    "target": "iOS",
    "target-version": "11.3",
    "target-device": "iPhone Simulator",
    "target-browser": "Safari",
    "appium-server-auth": "username:auth-token",
    "test-suite" : "renderjs",
    "remote-access-url": "https://softinst1234.host.vifib.net"
  }
we now use:
  {
    "target": "selenium-server",
    "desired-capabilities": {
      "platformName": "iOS",
      "browserName": "Safari",
      "platformVersion": "11.3",
      "deviceName": "iPhone Simulator"
    },
    "server-url": "https://username:auth-token@ondemand.saucelabs.com/wd/hub",
    "test-suite": "renderjs",
    "remote-access-url": "https://softinst106116.host.vifib.net"
  }

To run locally empty parameters are still OK.