ovn-northd, tests: Adding IPAM to ovn-northd.

ovn-northd, tests: Adding IPAM to ovn-northd.

Added an IPv4 and MAC addresses management system to ovn-northd. When a logical
switch's other_config:subnet field is set, logical ports attached to that
switch that have the keyword "dynamic" in their addresses column will
automatically be allocated a globally unique MAC address/unused IPv4 address
within the provided subnet. The allocated address will populate the
dynamic_addresses column. This can be useful for a user who wants to deploy
many VM's or containers with networking capabilities, but does not care about
the specific MAC/IPv4 addresses that are assigned.

Added tests in ovn.at for ipam.

Signed-off-by: Nimay Desai <nimaydesai1@gmail.com>
Acked-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Gurucharan Shetty <guru@ovn.org>