Update io.py

Add param "run_label" for create uniqe identifier output. This is necessary when a multithreaded program works with one server. Without additional unique identifier output mixes.

Example:
Without patch
from fabric.api import run, env, settings

for server in ("linuxbox", "linuxbox"):
    env.host = server
    with settings(host_string=env.host, warn_only=True):
        command = run("uname -s")

[linuxbox] run: uname -s
[linuxbox] run: uname -s
[linuxbox] out: Linux
[linuxbox] out: Linux


with patch:

from fabric.api import run, env, settings

k = 0
for server in ("linuxbox", "linuxbox"):
    env.host = server
    k += 1
    with settings(host_string=env.host, warn_only=True):
        command = run("uname -s", run_label=str(k))

[linuxbox, 1] run: uname -s
[linuxbox, 2] run: uname -s
[linuxbox, 2] out: Linux
[linuxbox, 1] out: Linux