[2.2, 2.3, API] Adding a CentOS image via CLI doesn't categorize it correctly

[Andres Rodriguez]

I've tried adding a centos image via the CLI like:

maas admin boot-resources create name=centos/centos7-105.2 architecture=amd64/generic content@=centos7

And the result is this:

1. On Images listing page:
 - See image under "Generated" images.
 - The name in the UI is shown as "centos/centos7-105.2"

2. Can't deploy image
 - I can't deploy the image (it is not available in the drop-down option).

The expected result of this is:

1. Images to show under "CentOS" section
2. Name to be show as "centos7-105.2"
3. Image to be available to deploy under "CentOS"

when adding a centos image with name=<os>/<name> it should assume that this image is a centos image.