raise exception when YAML file content not valid

Cloubed YAML file content has to be a dict once loaded.
raise exception if no storagepools in YAML
new method to check for all main keys in YAML

New method _check_main_keys() in Configuration to check the presence of all
main keys in the YAML file. Raise exception if error.
update README.md

Mention new cloubed script (with argparse dependency for Python 2.6) and
cosmetic s/ClouBed/Cloubed/.