Add support for principal secret

[m4ce]

When defining a principal, mesomatic should also support the principal's secret. This is needed if authentication is enabled on the mesos masters.

I believe this is achieved by using the Credentials.Builder:

public static final class Protos.Credentials.Builder
extends <any>
implements Protos.CredentialsOrBuilder

 Credentials used for framework authentication, HTTP authentication
 (where the common 'username' and 'password' are captured as
 'principal' and 'secret' respectively), etc.