test/nvme: increase delay between plugging devices

When the hotplug test was run in a qemu virtual machine, it consistently
failed to unplug the devices after insertion. Increasing the delay here
to four seconds resulted in proper deletion of the devices and a passing
test. (replicated three times)

Change-Id: I0283c8440568edd8dd8ca8945e691a29b775aab5
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/371401


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>