[r=dimitern] environs: Add AllocateAddress(instId, netId) call
This introduces a new Environ interface method which will
be used to allocate extra addresses for an instance. It is
the first step towards enabling container addressability for
EC2 and MAAS in a unified way.
Right now only the dummy provider has an implementation for
the new method (for testing), EC2 will be first to implement
it (in a follow-up, not to clutter the review).
https://codereview.appspot.com/91150044/
R=fwereade, natefinch