feat(mixins/SslCertificate): Let's Encrypt support

[fbeauchamp]

Test recipe

• create a xo, listenning to port 80 and 443
• expose this server to the internet
• ssl certificate should be broken ( missing/self signed)
• add certDomain = 'domainname.xxx' to the https part of the config
• reload the server
• load the https page
• it should have a valid certificate after a few seconds
• next loading should not re compute a certificate
• accessing XO through another domain (localhost) should show a self signed certificate
• accessing XO through the IP shoul show a self seigned certifiate
• redirect to https shouldn't redirect challenge

Check list

Check if done, if not relevant leave unchecked.

• PR reference the relevant issue (e.g. Fixes #007 or See xoa-support#42)
• if UI changes, a screenshot has been added to the PR
• documentation updated
• CHANGELOG.unreleased.md:
  • enhancement/bug fix entry added
  • list of packages to release updated (${name} v${new version})
• I have tested added/updated features (and impacted code)
  • unit tests (e.g. cron/parse.spec.js)
  • if xo-server API changes, the corresponding test has been added to/updated on xo-server-test
  • at least manual testing

Process

1. create a PR as soon as possible
2. mark it as WiP: (Work in Progress) if not ready to be merged
3. when you want a review, add a reviewer (and only one)
4. if necessary, update your PR, and re- add a reviewer

From the Four Agreements:

1. Be impeccable with your word.
2. Don't take anything personally.
3. Don't make assumptions.
4. Always do your best.