[Rackspace] Remove circular requires from Storage

The fog DSL automatically requires files when `models` and `collections`
have been declared.

When Metadata is required, it attempted to require both file, files,
directory and directories, both trying to require the same.

When fog/fog-core#32 was done, the ordering changed so that directory
was required first and referenced metadata which referenced Directory.

Fixes fog/fog#2825
Merge branch 'master' into rackspace_requires
Merge pull request #2829 from fog/rackspace_requires

[Rackspace] Remove circular requires from Storage
[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