Add caaIdentities entry to /directory endpoint's meta entry

[cpu]

We should add the "caa-identities" key to the "meta" key added to the "/directory" response in 8c54747. This will help subscribers know what value to publish in their DNS zone CAA records.

From the ACME-06 draft, Section 7.1.1, Directory:

  "caa-identities" (optional, array of string):  Each string MUST be a
  lowercase hostname which the ACME server recognizes as referring
  to itself for the purposes of CAA record validation as defined in
  [RFC6844].  This allows clients to determine the correct issuer
  domain name to use when configuring CAA records.

[rolandshoemaker]

Given there are a number of things that can (and probably should) be put in the meta field how would people feel about adding a metaDict field to the WFE config and just piping it directly into the directory object instead of having to have a bunch of separate fields/code paths to add things to the meta object?

[jsha]

Works for me.

[cpu]

Sounds good to me too.

[PovilasVytautasC]

Hello,

this pull request as I would image already went to staging environment. Since the /directory object has been changed it broke down one of the libraries (https://github.com/fszlin/certes) that use ACME protocol to connect to LE API. The reason for this is that previously all values in the return JSON were URIs. Now, the meta value is an object.

Wouldn't it be better to make metadata accessible in a separate endpoint not to affect the people who are currently using LE API?