Pin CDO to 1.8

At HLRN, cdo=1.9 with our pinned netcdf4=1.2.8 leads to missing shared HDF5 libraries. Pinning to cdo=1.8 works.

• Manual hot-fix: install cdo=1.8
  • Hannover
    • py2_std
    • py3_std
  • Berlin
    • py2_std
    • py3_std
  • Taurus
    • py2_std
    • py3_std
  • Nesh
    • py2_std
    • py3_std
• Pin in _env_templates

(cc: @sebastian-wahl )