Use local images

If the user has not specified a registry in their image then assume it's a locally tagged image.

This is useful in testing before you push to a registry
I like this idea.  I think it's more transparent if we make this an explicit flag though, rather than trying to detect this case. 

For example, a `bin/centurion` command line flag to ignore the registry and use only cached images would solve this case and also probably help out with other scenarios as well.  What do you think?