CSIT-1338 FIX: Failed to get VPP version on host

VPP was not initialized (shm), after service restart, quickly enough.
This results into vpp_api_test console timeouts when communicating over
uninitialized socket.

Patch is introducing more stable test to detect service (VPP) is fully
ready and initialized.

Change-Id: I300a763814feba28cb8163dcd4996b53b2994fec
Signed-off-by: Peter Mikus <pmikus@cisco.com>