UrnAlias for custom image

> ### `az feedback` auto-generates most of the information requested below, as of CLI version 2.0.74

**Describe the bug**
I have the following output:

root@Alexey-HP:~# az vm image list --publisher Canonical
You are viewing an offline list of images, use --all to retrieve an up-to-date list
Offer Publisher Sku Urn UrnAlias Version
------------ ----------- --------- --------------------------------------- ---------- ---------
UbuntuServer Canonical 18.04-LTS Canonical:UbuntuServer:18.04-LTS:latest UbuntuLTS latest
root@Alexey-HP:~#

where UrnAlias is UbuntuLTS.

I can create VM using UrnAlias:

az vm create --image UbuntuLTS ...

When I run the same command for my custom images there's no UrnAlias column (output is empty line):

root@Alexey-HP:~# az vm image list --publisher postgres-pro
You are viewing an offline list of images, use --all to retrieve an up-to-date list

root@Alexey-HP:~#

Instead I have to specify '--all' key to get a non-empty output of this command, but in this case the output is long enough.

**To Reproduce**
See above.

**Expected behavior**
How can I get UrnAlias column for my custom image?

or is there functionality of image family, so that I can group my custom images and create VM with simple command like az vm create --image UbuntuLTS ... with the latest version of my family image

**Environment summary**
Install Method (e.g. pip, interactive script, apt-get, Docker, MSI, edge build) / CLI version (`az --version`) / OS version / Shell Type (e.g. bash, cmd.exe, Bash on Windows)

**Additional context**
Add any other context about the problem here.