option to push local image to somewhere else

[deitch]

- What I did

Added option to lkt cache push foo to provide a remote tag as lkt cache push foo --remote-name bar, which means, "take the local image or index foo, and push it to the remote bar".

This is morally equivalent to the classical "tag&push", but much simpler

- How I did it

Code

- How to verify it

I did. I pushed and tested

- Description for the changelog

Ability to name images remotely differently