Support for consistent MAC address.

Right now, MAC addresses are randomly generated by the kernel when
creating the veth interfaces.

This causes different issues related to ARP, such as #4581, #5737 and #8269.

This change adds support for consistent MAC addresses, guaranteeing that
an IP address will always end up with the same MAC address, no matter
what.

Since IP addresses are already guaranteed to be unique by the
IPAllocator, MAC addresses will inherit this property as well for free.

Consistent mac addresses is also a requirement for stable networking (#8297)
since re-using the same IP address on a different MAC address triggers the ARP
issue.

Finally, this change makes the MAC address accessible through docker
inspect, which fixes #4033.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>
of course :P