Fix compatibility issue with Python 2.6
Fix `test_missing_options()` test removing too many options from the config file.

Since the `cfg` object was created *outside* the `missing_option` function, every time the function was called one *more* option was removed from the config file. This caused the test to always pass if the *first* option removed raised an error, even if other options were not raising the same error.
Fix test: ConfigParser().set() requires a string as a value, `bool` types are not allowed.
Fix tests: `ssh_to` option is not mandatory
Always raise a `MultipleInvalid` exception if some error is found in
the configuration file.
`<node>_groups` option in `setup` section is not mandatory. (Not sure this is correct though)
Add cluster section which uses the `google1` cloud provider. This is necessary, otherwise the validator will not check the validity of the `google1` cloud section.
Fix `test_read_missing_section_cluster` test: we need to remove *all*
the cluster sections from the config file in order to raise an error.