cjson:metadata should override auto detected metadata

[mhanysz]

Is your feature request related to a problem? Please describe.
Sometimes, especially when test reports are shipped to customers or are used to generate other reports (e.g. html reports or aggregated reports with reports from other tools), it may be helpful to override the automatically detected metadata values in the report. Customers or other tools may refer to MacOS versions as "Mojave" instead of 10.14 or to a specific browser version as the "LTS" or "embedded" version and so on.

Describe the solution you'd like
The metadata passed via cjson:metadata should override automatically detected values. If the developer goes the extra inch (you can hardly call it a mile) to specify cjson:metadata, it can be assumed that the passed data is carefully thought about and intended to show up in the report. Otherwise, the developer could just leave out parts of the cjson:metadata object or not specify it at all, to let auto detection handle the metadata.

Currently, only parts of the cjson:metadata object are optional. To make the approach described above more flexible, it might also be a good idea to make each individual part of the cjson:metadata object and it's nested objects optional. This way, the developer could rely on auto detection for all metadata except for the browser version, for example, by passing:

"cjson:metadata": {
  "browser: {
    version": "42"
  }
}

This isn't possible at the moment, because only the browser object itself is optional. If it's specified, it has to contain name and version.