Storage consistency

Use the `gophercloud.ServiceClient` within `openstack/storage/v1`.

I also:

 * Added `OkCodes:` to the perigee requests, so they'll fail more informatively if a non-successful HTTP status is returned.
 * Tweaked the openstack provider to normalize URLs from the service catalog with a trailing slash.
 * Updated the storage acceptance test to match the code changes.

This is the first part of #188.
I wasn't fond of the duplications among the various `urls.go` files, here. But, this lets us keep them private and keeps from snarling up the import dependencies again.

An alternative would be to introduce a `common/` subpackage and put them there, but I'm not sure I like that.