Implement allocating IPs from CIDR within bridge network 

Fixes #4986

It is similar to openstack nova `--fixed-cidr` and allows to do things like [this](http://goldmann.pl/blog/2014/01/21/connecting-docker-containers-on-multiple-hosts/). And our network engineer very excited about this change :)
Also I've refactored ipallocator a little more.
@LK4D4 Docs mostly LGTM. However, I find "[..] nested in bridge subnet" to be bit confusing.
Do you think there can be another way to put it so it becomes simpler and easier to understand?