labs: wait for namespaces and interfaces

When setting up neutron router and DHCP, the time until namespaces and
interfaces are ready varies a lot (from less than a second to five
seconds and more), even on the same host system. If it takes longer, we
proceed too early which leaves the cluster misconfigured. In that case,
nova instances fail to get a DHCP address.

This patch makes sure the script waits until the interfaces are ready.

Change-Id: I346a098062d984956cf496e89f9ada2a38df0569
Merge "labs: wait for namespaces and interfaces"