Enable multiple HTTP authenticator modules

To accommodate executor authentication, we will add support for the loading of multiple authenticator modules. The --http_authenticators flag is already set up for this, but we must relax the constraint in Mesos which enforces just a single authenticator.

In order to load multiple authenticators for a realm, a new Mesos-level authenticator, the CombinedAuthenticator, will be added. This class will call multiple authenticators and combine their results if necessary.