support hostname + base path rewrite

**Describe the problem/challenge you have**

jfrog supports a way to proxy image downloads from a particular hostname + base path (e.g. jfrog.corp.com/project-x proxies all requests to upstream-registry.com/some-path). it should be possible to rewrite lock file to rewrite lock entries.

**Describe the solution you'd like**

if user is using imgpkg bundles then i think there would be no need for this since bundle would be proxied and found in the virtual repo. .imgpkg/images.yml should be relative to that location.

should we have a feature in kbld that facilities rewriting hostnames, not sure...

**Anything else you would like to add:**

https://www.jfrog.com/confluence/display/JFROG/Docker+Registry#DockerRegistry-VirtualDockerRepositories ?