Apparently, fuel-agent doesn't handle any signal received except for SIGINT
which is automatically converted by python to KeyboardInterrupt() exception.
fuel-agent is unable to send signal for spawned processes, just because
utils.execute doesn't know PIDs of opened subprocessess. To mitigate that flaw,
fuel-agent will use process group to distribute signals.
Process groups are used to control the distribution of signals.
A signal directed to a process group is delivered individually to all of the
processes that are members of the group.
That allows fuel-agent to send signals to subprocesses without knowing thier