[builder] Builder should use dynamic channels

[chefsalim]

We need to be able to isolate groups of builds in order to not introduce instability in the build dependencies while groups of builds are in progress.

• The builder should upload builds to a newly created (transient) channel
• The package list page should show filter by channels (so we don't clutter up the space)
• hab studio should be able to have an option for pulling packages from a specified channel
• The builder should promote builds from the channel to the target channel, and delete the channel on group completion