Use closefds=True and shell=True when executing usercmds

Use closefds=True and shell=True when executing usercmds

Ticket: CM-1438
Reviewed By: reported by purna
Testing Done: Tested with purna's l2 l3 lag test

problem fixed by this patch:
In some cases the child processes executing user cmds seem to hold on to
the lock file fd for a lil longer, preventing another instance of
ifupdown from running immediately after. Seen with two immediate
instances of service networking restarts from scripts when the
interfaces file has many user cmds.