Add 4.3 to Current CI setup

[deeplow]

Description

Install 4.3 on the current CI to help flag the 4.3 compatibility status.

[deeplow]

This is being worked on under #1373

[deeplow]

How do we want to handle this one? (CC @legoktm). Do we want to start a 4.2 and 4.3 test at the same time? Or is there a better strategy resources-wise we can do here?

We could for example implement the "tags"-initiated OpenQA jobs where there is an OpenQA-4.2 and an OpenQA-4.3.

Then we could add a bypassable check that checks if we've run the PR at least on 4.2 for the time being. On 4.3-specific PRs we run also the 4.3 jobs and in the nightlies we could have both run.

[legoktm]

We could for example implement the "tags"-initiated OpenQA jobs where there is an OpenQA-4.2 and an OpenQA-4.3.

This sounds good to me and is a good opportunity to finally getting around to making that switch. (I'm not sure this needs to exactly block the 4.3 PR, e.g. you can run it through OpenQA and then back out the 4.3 part while we switch over to tags, but I'll leave the sequencing to you.)