Allow overriding the Cloudant URL when deployed to Bluemix

Sometimes it's useful to specify a Cloudant URL without binding to a Cloudant service directly (e.g. if you want to use a proxy).

This should be possible by specifying a URL using `SSS_CLOUDANT_URL` but  SSS always looks for a Cloudant service in `VCAP_SERVICES` if it's present, preventing `SSS_CLOUDANT_URL` from being used in Bluemix deployments.