Split workflow in several hooks

[La0]

Fixes #14

So we can work on #150

[marco-c]

Could you split it in actual separate Taskcluster hooks? One for cron, one triggered by pulselistener. Then, you could add an argument to cli.py to choose the workflow, to make it less brittle than relying on the values of other args (https://github.com/mozilla/code-coverage/pull/165/files#diff-6310014f6dc184cfe909a867e61bfcb8R64-R77).

Nit: I'd rename "mc.py" and "try_repo.py" to make it clear what they do (e.g. if we support other repos in the future, "mc.py" would not make sense anymore). "mc.py" is the task that runs at the end of a full test run, "try_repo.py" is the one that runs on-demand when developers push to try. I'm not sure what a good naming would be, maybe tests.py/all_tests.py/full_tests.py/push_tests.py/push.py/merge.py/merge_tests.py and on_demand.py?

[La0]

I went further than your comment, and used only 2 hooks:

1. code_review_bot.hooks.repo for all repositories, using a static configuration to change behaviour according to the repository in use
2. code_review_bot.hooks.cron for a scheduled task

I prefered to use 2 different endpoints, as some CLI options are not used by the cron worfklow.
The CLI & TC setup code is shared by both endpoints though.

In the end, i think the code is a bit cleaner, as we now only have 2 different worfklows (one being parametrizable), and we can support other repos a bit more easily.

[marco-c]

Great cleanup!