report packages from security pocket

[Impact]

I report this bug to add the necessary bit into lds-client for all affected/supported releases.

[Test Case]

* One must use Landscape server on-prem (version >=19.01) or hosted which already contain the necessary server side change.
* Install landscape-client.
* Successfully register a client against Landscape server.
* Security updates will only rely on USN notices and could possibly ignore other packages found in -security pocket even if they are there simply by the fact that there was no USN notice specific for them.

(e.g. systemd has an USN, systemd get updated but its derived systemd binary packages aren't updated)

[Regression Potential]

* The patch flags potential security updates by matching the pocket name. The server then does additional package selection from that info. If the pocket matching were to break, security updates would continue as it was previously.
* False positive matching could be possible, assuming one builds a mirror which mimics security pockets and contains normal updates. In that case, landscape could mistakenly update as if they were security updates.
* Landscape may apply security updates without USN data, if the update comes from a security pocket. Since this matches the behaviour of unattended-upgrades and MOTD info, this may be closer to what users expect, even though this is a change of behaviour.

[Other Info]

* Upstream details:
https://github.com/CanonicalLtd/landscape-client/commit/93a3b47965da199785e9b3d226cb61f721e54196
https://github.com/CanonicalLtd/landscape-client/pull/57
https://github.com/CanonicalLtd/landscape-client/pull/70