Restore behavior of trimming / from kubecfg url

Also make the output and validation of input better for kubecfg api calls.
Kubecfg will now display a usage argument if the URL is incorrect or an
unrecognized storage type is passed.
Merge pull request #581 from smarterclayton/fix_introduced_bugs_in_kubecfg

Restore behavior of trimming / from kubecfg url