scenario cross_tenant_connectivity

test conectivity between VMs in the same tenant and on different
tenants.
for each case, test that default state blocks traffic(*), that security
rules allow only specified traffic

(*)negative connectivity check is skipped until bug 1252620 is fixed
   TODO(yfried): enable them once bug is fixed

Also adds workaround for bug 1247568 in python-neutronclient (preventing
tearDownClass from ignoring deleted neutron resources

Change-Id: I212873d46e20ff3161e376105af18020b1cf1c9d
Merge "scenario cross_tenant_connectivity"