ovn: Allow for automatic dynamic updates of IPAM

OVN offers a method of IP address management that allows for an IPv4 subnet or
IPv6 prefix to be specified on a logical switch. Then by specifying a
switch port's address as "dynamic" or "<mac address> dynamic", OVN will
automatically assign addresses to the switch port.

While this works great for initial assignment of addresses, addresses do
not automatically adjust when changes are made to the switch's
configuration. For instance:
* If the subnet, ipv6_prefix, or exclude_ips for a logical switch
changes, the affected switch ports are not updated.
* If a switch port with a static IP address is added to the switch, and
that address conflicts with a dynamically assigned IP address, the
dynamic address is not updated.
* If a MAC address switched from being statically assigned to
dynamically assigned, the MAC address would not be updated.
* If a statically assigned MAC address changed, then the IPv6 address
would not be updated.

This patch solves all of the above issues by changing the algorithm for
IPAM assignment. There are essentially three steps.
1) While joining logical ports, all statically-assigned addresses (i.e.
any ports without "dynamic" addresses) have their addresses registered
to IPAM. This gives them top priority.
2) All logical ports with dynamic addresses are inspected. Any changes
that must be made to the addresses are collected to be made later. Any
addresses that do not require change are registered to IPAM. This allows
for previously assigned dynamic addresses to be kept.
3) All gathered changes are enacted.

The change contains new tests that ensure that dynamic addresses are
updated when appropriate.

This patch also alters some existing IPAM tests. Those tests assumed
that dynamic addresses would not be updated automatically, so those
tests either had to be altered or removed.

Signed-off-by: Mark Michelson <mmichels@redhat.com>
Acked-by: Jakub Sitnicki <jsitnicki@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>