Allow Framework Authentication Secret to be passed with as a path-to-file containing the secret, as well as parameter

[JohnOmernik]

Mesos, Marathon, Chronos all support having the Secret for Framework authentication be stored in a file.

Based on the help for the scheduler, it only lets us pass the secret (on the command line or in the .properties files)

--principal Principal (username) used to register framework. Default - none
--secret Secret (password) used to register framework. Default - none

Ideally, if we could do:

--secret Secret (password) used to register framework. Default - none
or
--secretfile Path to file containing the Secret (password) used to register framework. Default - none

With --secret taking precedence if both are presented, I think this would allow us register in the same way as marathon and chronos, while not breaking any existing implementations.