Make mirror configurable in spack_config

[adrienbernede]

Addresses #32

This is a basic proposal to address the issue.
However it is not protected against some cases, like if someone configures a mirror in the json file, and then another on command line.

[white238]

Unless I'm missing something, having the mirror.yaml not under the SYS_TYPE folder seems like it wouldn't work. For example, if I put /usr/workspace/some_shared_space/mirror like we normally do for LC, what happens on your personal machine? Does Spack have logic in there to toggle based on environment variables? You can in environment files but reading their documentation isn't helping me find it in the mirrors.yaml.

[cyrush]

@white238 in the wonderful universe of possibilities - we have https mirrors that work anywhere the internets work.

[white238]

Can you list mirrors and it just keeps going down the list if it can't access them?

[adrienbernede]

Of course we can do something more advanced. I was trying to adjust the effort to the minimum requirement.
In general, the way we handle the configuration lack some uniformity and robustness.
We could rely more on Spack commands now that they exists (like the one you showed me yesterday @white238).