* facts: Allow multiple entries per iface
There can be multiple addresses per interface, as well as multiple
neighbors.
This changes the format of "ansible_net_neighbors" to list instead of
dict, because the old format could not store multiple neighbors per
interface.
Also, this fixes a crash when the ipv6 module is not loaded, because the
error "bad command name" was being parsed with interface=None
* facts: Fix tests
* facts: Add changelog fragment