Renamed spacemacs-config-disabed-packages to dotspacemacs-excluded-packages

And make it usable for both packages and extensions.
Users should upgrade their .spacemacs file bu adding the definition of
this variable (dotspacemacs-excluded-packages), see .spacemacs.template
Document dotspacemacs-excluded-packages

And create Configuration section in readme
Add troubleshooting for new .spacemacs version