Don't pass task-related arguments to mesos-executor

TL;DR: When a command executor is used with shell=false and an array of arguments, those same arguments are directly passed to mesos-executor which fails miserably.

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Attempting to launch a task using the command executor with shell=false and passing arguments fails strangely.

command {
  value: "/my_program"
  user: "app"
  shell: false
  arguments: "my_program"
  arguments: "--start"
  arguments: "2014-10-06"
  arguments: "--end"
  arguments: "2014-10-07"
}

Dies with:

Failed to load unknown flag 'end'
Usage: my_program [...]

Supported options:
  --[no-]help         Prints this help message (default: false)
  --[no-]override     Whether or not to override the command the executor should run
                      when the task is launched. Only this flag is expected to be on
                      the command line and all arguments after the flag will be used as
                      the subsequent 'argv' to be used with 'execvp' (default: false)

This is coming from a failed attempt to have the slave launch mesos-executor. This is due to an adverse interaction between new CommandInfo features and this blurb from src/slave/slave.cpp:

    // Copy the CommandInfo to get the URIs and environment, but
    // update it to invoke 'mesos-executor' (unless we couldn't
    // resolve 'mesos-executor' via 'realpath', in which case just
    // echo the error and exit).
    executor.mutable_command()->MergeFrom(task.command());

    Result<string> path = os::realpath(
        path::join(flags.launcher_dir, "mesos-executor"));

    if (path.isSome()) {
      executor.mutable_command()->set_value(path.get());
    } else {
      executor.mutable_command()->set_value(
          "echo '" +
          (path.isError()
           ? path.error()
           : "No such file or directory") +
          "'; exit 1");
    }

This is failing to:

• clear the arguments field
• probably explicitly restore shell=true
• clear container ?
• clear user ?

I was able to quickly fix this locally by making a man-in-the-middle program at /usr/local/libexec/mesos/mesos-executor that stripped all args before exec-ing the real mesos-executor binary.