Add support for enabling/disabling mod_proxy_cluster/mod_proxy_balancer tests, split them in two in CI

[jajik]

close #398

• adds new variables to control mod_proxy_cluster/mod_proxy_balancer test execution
• simplifies the default initialization of variables
• separates mod_proxy_cluster and mod_proxy_balancer tests in CI so they run in parallel (saving around 20 minutes)

We have quite a lot of jobs and artifacts at the moment. I am going to rework it a little bit so that we waste fewer resources, but I'll do that in a separate PR to keep this easy to review.

[jajik]

@rhusar Unfortunately, this breaks the Required check again. Given the endless issues with this, I would suggest removing the Required tag for now, and I'll implement a different solution as part of the CI rework (I think the best way is to add a new Required job that will depend on all the others – if any job fails, it will fail too).

[rhusar]

@rhusar Unfortunately, this breaks the Required check again. Given the endless issues with this, I would suggest removing the Required tag for now, and I'll implement a different solution as part of the CI rework (I think the best way is to add a new Required job that will depend on all the others – if any job fails, it will fail too).

Agreed, it's annoying. The only reason it needed to be setup because other checks cannot be enforced without it...