Add support for ExecTask and ExecTaskStreaming APIs

[grembo]

The ExecTask API allows running commands within a running pot
as part of service checks, example:

service {
   check {
     type    = "script"
     command = "/bin/sh"
     args    = ["-c", "test -f /tmp/mycookie || exit 2"]
     interval = "10s"
     timeout = "5s"
   }
 }

The ExecTaskStreaming API allows running command within a pot
from the command line:

nomad alloc exec 94b77f14 touch /tmp/mycookie
nomad alloc -job myjob vi /etc/resolv.conf

This API also allows running commands and opening a shell using
the Nomad web UI.

[grembo]

@ebarriosjr @pizzamig Note: This is based on #31, I didn't figure out how to create proper stacked PRs. So once #31 landed, I'll rebase this one.

[grembo]

@pizzamig any chance to look into this one (and the signal one first)? They’re really helpful when dealing with nomad - especially being able to exec into a running pot using nomad’s web ui is a lot of fun.

[grembo]

Rebased now. Screenshot in action:

[pizzamig]

This is just awesome!