Added more options to getFiles so that you can set end_marker, prefix, path and delimiter in the openstack storage client.
Refactored the additional options using _.pick to reduce repetition. end_marker has been renamed to endMarker to conform with the option casing for pkgcloud.
Cleaning up inconsistent options handling for storage.getFiles

- Fixes a broken test as part of #334
Merge branch 'pr/334'