If, for some reason, the certs directory has permissions that are inaccessible by docker, we should still be able to fetch manifests using the `insecure` flag. Since the cli doesn't access the engine's list of insecure registries, the registry client should make a singleton list of the registry being queried with the `insecure` flag. Closes #1358 Signed-off-by: Christy Norman <christy@linux.vnet.ibm.com>