TEST-{20,30,31,40,50,60,70): wait for server interfaces to come up

TEST-{20,30,31,40,50,60,70): wait for server interfaces to come up

The network interfaces appear asynchronously and sometimes just too late,
after we're already halfway throught server-init.sh:

  + ip link set dev eth0 name ens3
  Cannot find device "eth0"
  + ip addr add 192.168.50.1/24 dev ens3
  Cannot find device "ens3"
  + dhcpd -cf /etc/dhcpd.conf -lf /var/lib/dhcpd/dhcpd.leases ens3 ens5
  ...
  [    8.040825] e1000 0000:00:03.0 eth0: (PCI:33MHz:32-bit) 52:54:01:12:34:56
  [    8.047105] e1000 0000:00:03.0 eth0: Intel(R) PRO/1000 Network Connection
  ...
  No subnet declaration for ens3 (no IPv4 addresses).
  ** Ignoring requests on ens3.  If this is not what
     you want, please write a subnet declaration
     in your dhcpd.conf file for the network segment
     to which interface ens3 is attached. **

Whoopsie. Let's ensure all the interfaces are there before we proceed
fiddling around with them.