Exercise multiple ansible versions in CI

[cristicalin]

What would you like to be added:
CI jobs for supported ansible versions.

Why is this needed:
We claim that we support ansible 2.9 and 2.10 (actually 3.x) and we also have requests to support ansible 2.11 (actually 4.x). We would need to cover these different versions in CI to be able to reliably know we did not inadvertently break a version we don't exercise in CI (like #8160).

My proposal is to move https://github.com/kubernetes-sigs/kubespray/blob/master/requirements.txt and https://github.com/kubernetes-sigs/kubespray/blob/master/tests/requirements.txt to a versioned requirements.txt (say requirements-2.10.txt) and link the default one to the requirements.txt and add for each supported version a requirements-2.x.txt versioned file. In addition to this we would need to modify https://github.com/kubernetes-sigs/kubespray/blob/master/tests/scripts/testcases_prepare.sh to allow test cases to specifiy the ansible versions and add at least one job for each supported ansible version with a default aio config.

[floryut]

Not a bad idea at all, and indeed not that much of work