[Fullstack] SG test - first wait until all vms can ping each other
[Fullstack] SG test - first wait until all vms can ping each other

In fullstack security group test, after fake VMs are created there
check if connectivity to some custom port is working fine with port
security disabled.
After that there is called "block_until_ping" method for each vms.

This patch changes that to first wait if we can ping vms and later do
netcat tests.
Even if that will not solve problems with failures of this test, we
may know more if the issue is caused by netcat or it's just no
ICMP connectivity between VMs at all.

Change-Id: Ie9e2170c761c9a10f3daa991c3fb77f304bb07e2
Related-Bug: #1742401

mahak/neutron,mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,openstack/neutron