Automatic merge from submit-queue

rkt: Convert image name to be a valid acidentifier

**Release note**:
<!--  Steps to write your release note:
1. Use the release-note-* labels to set the release note state (if you have access) 
2. Enter your extended release note in the below block; leaving it blank means using the PR title as the release note. If no release note is required, just write `NONE`. 
-->
```release-note
Fix a bug under the rkt runtime whereby image-registries with ports would not be fetched from
```

This fixes a bug whereby an image reference that included a port was not
recognized after being downloaded, and so could not be run

This is the quick-and-simple fix. In the longer term, we'll want to refactor image logic a bit more to handle the many special cases that the current code does not, mostly related to library images on dockerhub.

/cc @yifan-gu @kubernetes/sig-rktnetes