Custom conda channels

[chaosphere2112]

This goes with readthedocs/readthedocs.org#2507.

That PR adds custom conda channel support to the config parser; this one adds validation for those config options. It also adds tests for the validation.

I'm intentionally curtailing some of the functionality of conda here. conda -c $CHANNEL has support for 3 different kinds of input; here's the relevant help section:

  -c CHANNEL, --channel CHANNEL
                        Additional channel to search for packages. These are
                        URLs searched in the order they are given (including
                        file:// for local directories). Then, the defaults or
                        channels from .condarc are searched (unless
                        --override-channels is given). You can use 'defaults'
                        to get the default packages for conda, and 'system' to
                        get the system packages, which also takes .condarc
                        into account. You can also use any name and the
                        .condarc channel_alias value will be prepended. The
                        default channel_alias is http://conda.anaconda.org/.

It seems like a needless security issue to provide any of the valid inputs as working options here. Any time a system exposes its filesystem to open web handling I get itchy under the collar. So, I opted to limit valid channels to only being ones hosted at the channel_alias (conda.anaconda.org, presumably, though your deployment may vary). I did my best to tighten up the testing and validation on those checks, though I'm sure I missed something.

[humitos]

Closing because we are not sure we really need this. Read: readthedocs/readthedocs.org#2507