For the moment, I've removed the ability to specify
a dict of options without using **. This is a slightly
unfortunate trade-off since it simplifies implementation
at the expense of making the API slightly less convenient
(if somewhat more consistent.)

rt is still somewhat broken.

Ultimately we may wish to create a mininet/ cgroup and do a recursive
delete at the end.

Possibly this should be in intf() instead, as intf() is assumed
to always succeed.

- keys are strings
- metadata is simply a dict
- buildFromTopo greatly simplified