The zone DB no longer considers duplciates of (IP, name); it will
reject duplicates of (IP, name, ident), but this ultimately doesn't
matter since those are all the fields it records anyway.
The HTTP interface treats these duplicates as trivial success (i.e.,
it acts as if it's idempotent, which it is).
This changes the behaviour for removals: if (IP, name) is recorded for
two different containers, removing one will leave the other extant.
These two testing utils assumed that they got an error, and checked
its type; however, the most likely failure case is that there's no
error (i.e., nil). In that case they would cause a NPE panic.
e.g. when it was started with --net=none
weavedns wasn't actually using the docker-assigned ip for anything, so
this change is a simple matter of eradicating it from the code.
Michael Bridgen