Integration domain: de-duplicate code between AD and LDAP

[flo-renaud]

Integration domain: users_dn is a full DN

The config parameter users_dn is documented as
"Optional full DN of LDAP tree where users are"
but the current implementation is not consistent with the description.

Make this parameter mandatory, and expect it to contain the full
DN of the users tree, for instance:
ou=users,dc=example,dc=com

With this new behavior, no need to build the user dn from
relative dn + suffix.

Integration domain: de-duplicate code between AD and LDAP

The classes are 95% identical.
The current differences so far:

• the expected naming convention: users in AD are named
  cn=,$USER_BASE_DN
  while users in LDAP are named
  uid=,$USER_BASE_DN

• the bind DN (client_id) used to connect to the remote AD/LDAP
  server. For AD the code is using client_id@domain but we can
  use a user DN instead.

Introduce a class variable _user_rdn_attr that stores the
RDN attribute used to build a user DN.

Integration domain: allow 255 chars for client_id

The client_id contains a bind DN and can be longer than 20 chars.

[f-trivino]

@flo-renaud thanks for your PR. This considerably improves the code. I've completed e2e tests for AD and LDAP use cases using the following Payloads:

LDAP:

{
  "name": "ldap.test",
  "description": "LDAP Integration Domain",
  "integration_domain_url": "ldap://rhds.ldap.test",
  "client_id": "cn=Directory Manager",
  "client_secret": "Secret123",
  "id_provider": "ldap",
  "user_extra_attrs": "mail:mail, sn:sn, givenname:givenname",
  "user_object_classes": "",
  "users_dn": "ou=people,dc=ldap,dc=test",
  "ldap_tls_cacert": "/etc/openldap/certs/cacert.pem"
}

AD:

{
  "name": "da.test",
  "description": "AD Integration Domain",
  "integration_domain_url": "ldap://ad.da.test",
  "client_id": "[email protected]",
  "client_secret": "Secret123",
  "id_provider": "ad",
  "user_extra_attrs": "mail:mail, sn:sn, givenname:givenname",
  "user_object_classes": "",
  "users_dn": "cn=Users,dc=ldap,dc=test",
  "ldap_tls_cacert": "/etc/openldap/certs/cacert.pem"
}

AD and LDAP writable interface is able to add users in the integration domain, accordingly.

I'm approving and merging.

[f-trivino]

LGTM.

[f-trivino]

no makemigrations are needed for this change so I'm merging it.