Tests proxy configuration

[alexandre-allard]

Component: tests, ci

Why this is needed:
To ensure that everything works fine when a proxy is set

What should be done:
We need to test the setting of an http(s) proxy during the metalk8s multiple nodes installation scenario.

Implementation proposal (strongly recommended):
Just add a dummy proxy (pointing to a non-existent address) in the bootstrap.yaml configuration

proxies:
  http: http://10.10.10.10:1234
  https: https://10.10.10.10:1234

Related to: #2052

[gdemonet]

I'd argue it would be more useful to set up a realistic env, where proxy sits between containerd and the Internet, and we use it to fetch an arbitrary image (crictl pull -d gcr.io/etcd:3.4.3 for instance) in addition to one from our internal registry (that the proxy cannot resolve, by design).