Allow overriding the Cloudant URL when deployed to Bluemix

[willholley]

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.