ovn: Add address set support.

ovn: Add address set support.

Update the OVN expression parser to support address sets.  Previously,
you could have a set of IP or MAC addresses in this form:

    {addr1, addr2, ..., addrN}

This patch adds support for a bit of indirection where we can define a
set of addresses and refer to them by name.

    $name

This '$name' can be used in the expresssions like

    {addr1, addr2, $name, ... }
    {$name}
    $name

A future patch will expose the ability to define address sets for use.

Signed-off-by: Russell Bryant <russell@ovn.org>
Co-authored-by: Babu Shanmugam <bschanmu@redhat.com>
Signed-off-by: Babu Shanmugam <bschanmu@redhat.com>
[blp@ovn.org made numerous small changes]
Signed-off-by: Ben Pfaff <blp@ovn.org>