Add workflow to update environments collections

As Raphaël said in #339 (comment 474004):

We could also build an "UpdateEnvironments" workflow as the first workflow. Given a vendor, a list of codenames, a list of architectures, optionally a list of mmdebstrap/simplesystemimagebuild templates, it should schedule all the corresponding "mmdebstrap" and "simplesystemimagebuild" work requests and then record the resulting artifacts in the corresponding debian:environments collection.

This would give the opportunity to use the "groups of tasks" right away so that all tasks for a given release are displayed in a specific group.

Also note that different codenames have different lists of architectures, so I suspect that instead of a list of codenames and a list of architectures we would in fact want something like this:

vendor: debian
targets:
  - codename: jessie
    architectures: [...]
  - codename: stretch
    architectures: [...]
# plus whatever we need for build templates

Once we have this, we should also update the "Getting started with debusine" tutorial to use it rather than having to copy environment artifact IDs around.