This commit adds a new dcos task exec subcommand to the DC/OS CLI.
Using dcos task exec, one can launch arbitrary commands inside a task's
container in a DC/OS cluster. This includes commands that simply print
output to the screen (e.g. ls -la), run interactively (e.g. cat), or
take full control of the terminal (e.g. vim or bash).
Author attribution: Steven Locke @StevenLocke