Fix extension of conf.yaml file

[alistairsellar]

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Closes #432

[alistairsellar]

@LisaBock I think I've found a small error in the configuration episode. Would you be the right person to review this?

[LisaBock]

@alistairsellar Why do you think it's incorrect? Both endings yml and yaml could be used. And all configuration files in ESMValCore end with yml. So this is consistent.

[alistairsellar]

@alistairsellar Why do you think it's incorrect? Both endings yml and yaml could be used. And all configuration files in ESMValCore end with yml. So this is consistent.

Hi Lisa, as far as I can tell, intake-esgf uses a different convention from ESMValCore. I've put some more detail in the issue to explain my diagnosis of the problem.