feat(db) move ssl_certificates from dao to db

[kikito]

Migrations

• ssl_certificates table renamed to certificates
• ssl_servers_names table renamed to server_names. Added a new field (id, UUID) and made that the PK. But also added indexes for the name field.
• In postgresql these changes involved altering the tables. In Cassandra the changes involved copying all the records from the original tables to new tables (this was done with a custom migration helper which has its own PR).

Admin API

• /snis is now /server_names and is automatically generated by the new DAO admin api generator-from-schema.
• /certificates is also generated, but it has some overrides, to deal with three "special" things:
  • the server_names accepts a comma-separated string or an array of strings. It
    allows "setting the server names in one go" when creating (POST) or updating
    (PATCH) a certificate.
  • the server_names attribute is automatically added to the certs when getting
    each one individually or when getting a list of them.
  • a certificate can be referenced by its id or also by the name attribute of
    any of its associated server_names.
• PUT is not allowed any more.

DAO

• ssl_certificates and ssl_servers_names' schemas translated to new DAO's schemas (renamed to certificates and server_names).
• I had to include a read-before-delete in all the generated dao:delete* methods. This read was needed in order to propagate the structure through the worker events so it could be deleted properly (worker events can use any attribute
  from the entitity, for example for the server names you need their name attribute. So if you delete a server name using its id you still need to read it in order to propagate an event with the name field).
• certificates has a custom DAO to deal with the server_names pseudo-attribute and for "looking for a certificate using one of its server names' name"
• server_names also has a custom DAO for "using the server names as if they were lists of strings", which is easier to do than "handling them as instances of the server_names schema".

Tests

• the DAO:delete* methods now returns the entity which was erased instead of true, so some tests had to be changed from is_true to is_truthy.
• the tests for PUT where just removed
• I also removed the admin api tests from spec-old-api because they were a carbon copy of the new ones (there are no APIs, Services or Routes involved in SSL). I double-checked that we don't do anything "extra" on it before removing.

Future work

• The tests on this PR are the equivalent to the ones that existed before (and were not removed like the PUT ones). However, the auto-generated APIs include more endpoints which will also need to be tested. That will be done in a separate PR.
• I'm almost certain that the /server_names endpoint is not needed any more (it's useful for counting the total number of server names in some tests, but that's all). I will also explore the possibility of not including this endpoint on the other PR.
• And of course the migrations need to be tested as well. That'll be yet another PR.
• Since we're doing the read-before-delete on the DAO we probably don't need the delete_by_field methods on the strategies (we can use the PK, we have it because of the "read"). We could remove those methods.

[kikito]

In order to simplify things, I'm going to close this in favor of #3386 (which contains all the commits on this + more tests)