Build only one flavor on PRs

[jimmykarily]

We don't have unlimited CI resources and currently each PR takes ages to build all flavors just to run some tests on one of them and some on another.

Accepting the fact that we can't have everything in life, we will:

• Build only one flavor on RPs and run all tests on that flavor
• Build all flavors on master and run tests like we do now (some tests on one, some tests on another)

[mauromorales]

I see that on PRs we only use Ubuntu, but on master we test sometimes multiple flavors and sometimes only opensuse-leap, should the latter also be ubuntu?

cc @jimmykarily @mudler

[jimmykarily]

In PRs we want to avoid building more than one flavors, to speed up things. If we have to use only one flavor for all, that has to be Ubuntu because we removed zfs support from OpenSUSE at some point (flaking repo or something) so the zfs test won't succeed there.

On master, we build all the flavors anyway so there is no reason to use Ubuntu for everything. In contrary, if we run some tests on OpenSUSE we may catch some bugs that wouldn't be caught on the PRs (which run on Ubuntu).

As the description above says, we shuffle things a bit on master in a hope to catch problems.

[mauromorales]

Ok makes sense, maybe we should have this documented somewhere? could be as part of #1527 ?