Simplify configure task in recipe test workflow

[alistairsellar]

Is your feature request related to a problem? Please describe.
At present, the configure task in RTW reads the example user config file from the repo and overwrites with any variables provided by the workflow config (mainly DRS). However, ESMValTool doesn't require a user config file to contain all the keys specified in the user file, and will provide defaults for any keys not in the file (indeed the example file doesn't include all possible config variables). It therefore will be simpler to write only the keys provided by the RTW.

Part of #2786

Would you be able to help out?
Yes

[alistairsellar]

Completed by #3394