ovn: Rename "gateway" to "l3gateway".

ovn: Rename "gateway" to "l3gateway".

When L3 gateway support was added, it introduced a port type called
"gateway" and a corresponding option called "gateway-chassis".  Since
that time, we also have an L2 gateway port type called "l2gateway" and a
corresponding option called "l2gateway-chassis".  This patch renames the
L3 gateway port type and option to "l3gateway" and "l3gateway-chassis"
to make things a little more clear and consistent.

Signed-off-by: Russell Bryant <russell@ovn.org>