nfvbenchvm: refactor wait for VPP service

Sleeping for 10s and hoping that VPP service will be ready to accept
vppctl commands does not work all the times.  Instead of increasing the
sleep time, poll every second until "vppctl show int" returns something.
Wait at most 30 seconds.

By the way, exit with a clear error message if "vppctl show int" does
not report two network interfaces as expected.

Change-Id: I6f0f053378c0bee8fd28a231dc1041c94642f5b3
Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>