When using repo discovery to pull containers from Docker.io registry, containers from main namespace (without slash in name) will fail to sync afterwards.

The problem is that while repo discovery works against `index.docker.io`, containers must be properly namespaced to be pulled from it. So instead of `alpine`, `library/alpine` must be pulled.

It seems that `library` is implicit when pulling images from `registry-1.docker.io`, so just `alpine` will work; but repo discovery doesn't work against that host, which means manual creation of repositories.

While this might be problem on Docker.io side, they are the biggest provider of images out there, so we might want to adjust to them nevertheless.


Version-Release number of selected component (if applicable):
Sat 6.5 snap 2
katello-3.9.0-0.11.rc2.el7sat.noarch
satellite-6.5.0-3.beta.el7sat.noarch
foreman-1.20.0-0.17.RC2.el7sat.noarch


Steps to Reproduce:
1. Content -> Products -> Repo discovery -> Type: Container images
2. Registry: index.docker.io -> Registry Search Parameter: alpine
3. Select `alpine` and `alpine/git` -> Create Selected -> Run repository creation
4. Navigate to product page, sync both repositories

Actual results:
Syncing `alpine` repository fails; syncing `alpine/git` succeeds.
To make alpine sync, go to details page and do either of following:
- change Registry URL to `https://registry-1.docker.io/`
- change Upstream Repository Name to `library/alpine`


Expected results:
Syncing both succeeds.