Control compilers through programming environments

[tkoskela]

At the moment we always build in the default programming environment and let spack choose the compiler it defaults to. This is problematic because the user has no easy way of a) knowing which compiler is being used and b) choosing the compiler they want. The only way to select the compiler is through changing the spack spec on the command line. This then overrides the whole spec defined in the reframe class including versions, variants etc. unless the user is aware of them.

One way to fix this issue would be to specify the programming environment for each system in ReFrame and pass the compiler to spack. I've discussed this with @giordano and he pointed out there are also some issues with this approach. One of them is that ReFrame by default tries to build and run in all valid programming environments and this has to be filtered on the command line, adding more burden on the user.

We need to think about this a bit more. Ideas are welcome