doc: fix IPSec experiments instructions

The vppctl program does not wait for completion before returning.
The code segments with dependencies are split to avoid failure due
to timing.

Fixes: 508322b0 ("doc/usecase: add IPSec usecase")

Signed-off-by: paul szczepanek <paul.szczepanek@arm.com>
Reviewed-by: Dhruv Tripathi <dhruv.tripathi@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>

Change-Id: Ie351d9a8194df3a0238bf7eb34bff83c4cd3295a