Support backend control via CLI

[tgr]

The attached patches (for trunk and for 2.1, the only difference is Sphinx docs) are a PoC of CLI-based backend control for round-robin and random directors. It's a quick hack and the code may not be where you want it, but it works.

Rationale:

- it may be desirable to allow application administrators to disable/enable backends without allowing VCL modification,

- there is presumably no need to disable a backend that is not part of a director,

- health probes should continue when a backend is disabled - it is easy to make a decision to disable/enable a backend based on its health status, when presented in a format like:

director1 backend1 Enabled Healthy
director1 backend2 Disabled Healthy
director1 backend3 Disabled Sick

[phk]

It is a good idea, but it runs into the same naming issue as many other backend related features and therefore I will not take the patch as is.

I have moved this to the PostTwoShoppingList with the other change requests.

I have promised myself to get that naming issue sorted out before 3.0

[tgr]

You're right. The naming issue is definitely something to ponder on. But meanwhile, if anybody needs this feature bad (like us), there's an interim patch that just disables backends in all copies of a director. Obviously, it's unsupported, etc.