Add function to generate rootpath entry for user config

[stefsmeets]

This PR adds a function to get the rootpath structure for the user config.
This is needed for ESMValGroup/ESMValCore#907

In [1]: from esmvaltool_sample_data import get_rootpaths
In [2]: get_rootpaths()
Out[2]:
{'rootpath': {'CMIP6': ['/home/stef/r/esmvaltool_sample_data/esmvaltool_sample_data/data/timeseries/CMIP6']},
 'drs': {'CMIP6': 'default'}}

Addressed

[nielsdrost]

Thanks @stefsmeets for taking care of all the comments by @bouweandela!