Add acme-buddy support

[weshinsley]

This PR puts the config and container creation code for acme, which we use in many places, into acme.py, so a typical deployment might look like

mrc-ide/packit-deploy#32

essentially calling the constellation functions
acme_cfg = config.config_acme(dat) and
acme_container = acme.acme_buddy_container(cfg, proxy, tls_volume)

It implements also the suggestions in vimc/montagu-deploy#22 regarding the different environment variables needed for different providers (HDB vs Cloudflare)

[codecov]

Codecov Report

❌ Patch coverage is 98.79518% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.29%. Comparing base (9973b1b) to head (4c3c523).
⚠️ Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
src/constellation/config.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   97.22%   97.29%   +0.06%     
==========================================
  Files          14       16       +2     
  Lines        1588     1661      +73     
  Branches       98      101       +3     
==========================================
+ Hits         1544     1616      +72     
  Misses         35       35              
- Partials        9       10       +1     

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[EmmaLRussell]

I guess the main test of this will be in the deployments that use it, as it's difficult to test without a real domain. This repo isn't really set up for integration tests. Or documentation - but I guess the test is a good example to work from..

[EmmaLRussell]

Why is this an env var and not a config value? Is the idea that this allows you to test "real" configs with a fake acme buddy set up?

[weshinsley]

Good question - in acme buddy, either the --staging flag or existence of a non-empty ACME_BUDDY_STAGING environment var achieve the same purpose - perhaps in constellation I should accept it as optional in both the config, and the environment...

[weshinsley]

Actually, I treat the env part of the config as a block, whatever it contains, so ACME_BUDDY_STAGING can be passed in as acme_buddy: env: ACME_BUDDY_STAGING. If the environment variable is present, then that overwrites the config, which is I think the right way round.