[Impact]
when users are getting the message update-notifier message through apt-check they may find inconsistent behavior regarding ESM products. This is misleading since we will say to the users that they don't have ESM Infra, but they do have ESM infra packages that can be installed. This is poor marketing of our products

[Test case]

To reproduce the issue, you can:

1. Launch the following old version of a xenial container:
   lxc launch ubuntu:f4c4c60a6b752a381288ae72a1689a9da00f8e03b732c8d1b8a8fcd1a8890800 dev-x

2. Run apt update and install the updated version of update-notifier-common
3. Add the ubuntu-advantage-tools ppa:
   https://code.launchpad.net/~ua-client/+archive/ubuntu/daily
4. Install ubuntu-advantage-tools
5. Install the latest version of uaclient from the stable ppa:
   https://launchpad.net/~ua-client/+archive/ubuntu/stable/
6. Comment out all mentions of xenial-security in /etc/apt/source.list
7. Run apt update
8. Run /usr/lib/update-notifier/apt-check --human-readable
9. See a message like this:
   
UA Infra: Extended Security Maintenance (ESM) is not enabled.

256 packages can be updated.
5 of these updates are fixed through UA Infra: ESM.
5 of these updates are security updates.
To see these additional updates run: apt list --upgradable

Enable UA Infra: ESM to receive 5 additional security updates.
See https://ubuntu.com/security/esm or run: sudo ua status


To verify that the error is fixed:

1.Perform all the stages above until step 8
2 Install the new update-notifier from this ppa:
  https://launchpad.net/~lamoura/+archive/ubuntu/update-notifier-test-ppa
3. Run /usr/lib/update-notifier/apt-check --human-readable
4. See a message like this:

256 updates can be installed immediately.
5 of these updates are security updates.
To see these additional updates run: apt list --upgradable

5. We are now only showing ESM infra specific message if the distro is ESM. To enforce that behavior, make the `is_esm_distro` function in `/usr/lub/update-notifier/apt-check` return True, then you will see this message:

UA Infra: Extended Security Maintenance (ESM) is not enabled.

256 updates can be installed immediately.
5 of these updates are security updates.
To see these additional updates run: apt list --upgradable

5 additional security updates can be applied with UA Infra: ESM
Learn more about enabling UA Infra: ESM service at https://ubuntu.com/esm

That is now correct.

[Where problems could occur]

The changes in this package should only be seen when MOTD is getting a new message. If that script fails for some reason, it seems that MOTD will only not present the message, which is doesn't seem to be a system critical issue. Additionally, we would potentially have tracebacks in the update-notifier logs. Finally, if the logic is also incorrect, we would be displying incorrect ESM messages to the user. But since we are doing this now, as this bug shows, I don't think this is critical as well.

[Discussion]

With ESM Apps going to production soon, we have decided to update the messages delivered by update-notifier apt-check to address the package count of ESM Apps and the possibility of installing more upgrades if the user has ESM Apps disabled.

We are also updating other parts of the messaging as well. First, we only display ESM Infra status
on ESM distros. However, we will keep showing the ESM Infra package count on all of them.

For ESM Apps, we are only performing the alerts (For example, that you might have x packages updates if ESM Apps is installed) if the user is on a LTS distro.

Since we going to perform that change, we decided to also address this bit in the SRU, since it could harm the message we are delivering