Changed Authenticatee to embed pid in AuthenticateMessage.

Review: https://reviews.apache.org/r/14508
Added authentication support to scheduler and master.

Review: https://reviews.apache.org/r/14292/
Improved MesosSchedulerDriver initialize() method to properly handle
authentication credentials.

Review: https://reviews.apache.org/r/14453
Updated all tests to do authentication.

Review: https://reviews.apache.org/r/14509
Added authentication tests.

Review: https://reviews.apache.org/r/14510