Add `stack setup --install-cabal VERSION` in addition to `stack setup --upgrade-cabal`.

[sjakobi]

This should make it easier to test stack with different versions of Cabal, as required for e.g. #2350.

[sjakobi]

A few pointers:

• Most (all?) of the related code lives in either Stack.SetupCmd or Stack.Setup.
• I'd suggest generalizing Stack.Setup.ugradeCabal to installCabal so it can accept a concrete version or default to the newest available version of Cabal

[Blaisorblade]

What about picking a fixed cabal-install version for each stack version, and installing it in isolation? Ideally, the newest one for the supported cabal library.
(Why is stack not yet on Cabal 1.24 BTW?)

[borsboom]

Stack doesn't use cabal-install for much (just the solver AFAIK) so not really any need to have cabal-install match the Cabal library version it was built with. For building Stack, we stick with the versions of packages that are in the current LTS snapshot unless there's a really compelling reason to add an extra-dep, so once LTS 7 is released Stack will upgrade to a newer Cabal library version (along with building with GHC 8.0.1). And we haven't been in a rush to have Stack use Cabal-1.24 because we don't need any of its new features.

[decentral1se]

Can be closed off now that #2995 went in (forgot to close with commit).

[mgsloan]

Indeed! Closing