[Brightbox] Test error when required args missing

This adds a test to confirm existing behaviour that ArgumentError is
raised when `brightbox_client_id` or `brightbox_secret` is omitted.
Merge pull request #2830 from fog/test_required_args

[Brightbox] Test error when required args missing
Add Rubocop and Fuubar to development dependencies
Add Rubocop config files
Rubocop: allow Void enforcement
Fix some errors and dead code caught by void-context warnings
Rubocop: allow EndAlignment enforcement
Correct misaligned "ends" caught by Rubocop
Rubocop: allow AmbiguousOperator enforcement
Fix a couple of ambiguous uses of &block parameters
Rubocop: allow ConditionPosition enforcement
Fix "single line" conditionals that were broken into two lines
Rubocop: allow StringConversionInInterpolation enforcement
Remove superfluous #to_s usage in interpolated strings
Rubocop: allow DeprecatedClassMethods enforcement
Replace "deprecated" class method File.exists? with .exist?