Allow multiple data sources in the RTW

[sloosvel]

Currently, the config-user.yml file that gets generated by the RTW in DKRZ only looks at one directory when finding the data for CMIP5 and CMIP6:

DRS_CMIP5="DKRZ"
DRS_CMIP6="DKRZ"
ENV_NAME="experimental"
KGO_ROOT_PATH="/work/bd0854/KGO/esmvaltool_output/"
ROOTPATH_CMIP5="/work/bd0854/DATA/ESMValTool2/CMIP5_DKRZ"
ROOTPATH_CMIP6="/work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ"
ROOTPATH_OBS="/work/bd0854/DATA/ESMValTool2/OBS"
ROOTPATH_OBS4MIPS="/work/bd0854/DATA/ESMValTool2/OBS"
SITE="dkrz"
       

Whereas data in DKRZ is spread over multiple directories:

rootpath:
  CMIP6:
    /work/bd0854/DATA/ESMValTool2/CMIP6_DKRZ: DKRZ
    /work/bd0854/DATA/ESMValTool2/download: ESGF
  CMIP5:
    /work/bd0854/DATA/ESMValTool2/CMIP5_DKRZ: DKRZ
    /work/bd0854/DATA/ESMValTool2/download: ESGF
  CMIP3:
    /work/bd0854/DATA/ESMValTool2/CMIP3: DKRZ
    /work/bd0854/DATA/ESMValTool2/download: ESGF
  CORDEX:
    /work/ik1017/C3SCORDEX/data/c3s-cordex/output: BADC
    /work/bd0854/DATA/ESMValTool2/download: ESGF
  OBS: /work/bd0854/DATA/ESMValTool2/OBS
  OBS6: /work/bd0854/DATA/ESMValTool2/OBS
  obs4MIPs:
    /work/bd0854/DATA/ESMValTool2/OBS: default
    /work/bd0854/DATA/ESMValTool2/download: ESGF
  ana4mips: /work/bd0854/DATA/ESMValTool2/OBS
  native6:
    /work/bd0854/DATA/ESMValTool2/RAWOBS: default
    /pool/data/ERA5: DKRZ-ERA5-GRIB
  RAWOBS: /work/bd0854/DATA/ESMValTool2/RAWOBS

This makes recipes in the RTW fail due to missing data, when the real issue is that not all directories are being taken into account when finding the data.