[openstack storage] update getFiles() to return all container files by default (>10k)

Cloudfiles will return a maximum of 10,000 files at once when listing a containers contents. Previously, listing a container with >10,000 objects required a pkgcloud user to use the `limit` and `marker` options to aggregate results. This update causes pkgcloud's `openstack.getFiles()` method to internally request additional files if 10,000 results were returned in the previous GET request.

The behavior of `getFiles()` remains unchanged if the user passes a `limit` or `marker` option.