Add some robustness: Be more paranoid about trusing the switch's

return value on VLAN creation. If it reports success, we look for the
VLAN with findVlan(), to make sure it's really there. We retry a
couple times if it didn't work. This will add a small (sub-second)
time penalty per VLAN created.

We ran into one instance, that we've noticed, where the switch
reported that VLAN creation succeeded, but the VLAN did not actually
get created on the switch.