add a synchronous function with timeout to execute command

We need a common function to execute shell command.
We also need timeout when execute shell command.

A threading.Timer is used to send signal.SIGKILL to kill the command
when timeout.

run a command with timeout as follow.
$ PYTHONPATH=src python -c '
from kimchi import utils
print utils.run_command(["dd", "if=/dev/zero", "of=/dev/null"], 1.5)
'

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>