Create CI test for Ubuntu 18.04 stock image

[techcet]

• Would like to have more coverage for recommended OS's with a stock kernel version new enough to run Cilium

cc @eloycoto

[ianvernon]

What would this stock kernel version be? We are using 4.9.17-040917.201703220831 in the CI because it is the minimal kernel version we support. I agree though that testing against multiple kernel versions is a good idea. The only reservation I have for this is that it increases the number of tests to triage.

[techcet]

Let's hold temporarily on this. I'd like to have a test but it might not have to be a CI test. I believe the stock kernel version for 18.04 is 4.15.

[ianvernon]

@techcet sounds good!