Add a stevedore plugin interface to support pluggable authentication methods, to support current authentication methods as well as future implementations by wazo, partners or community.

The new plugin will be used for the bulk of the authentication mechanism, which is executed when a user attempts to login through a POST /tokens request.

The login requests proposes login credentials according to the authentication method(e.g. username/password for native auth, username/password/domain for ldap, saml session id for SAML), the authentication mechanism is selected based on that request, the authentication mechanism validates the credentials/login request, and a token is created as a witness of this authentication for subsequent authenticated and authorized interactions.