ovn-sb: Remove the "Gateway" table from the ovn-sb schema.

ovn-sb: Remove the "Gateway" table from the ovn-sb schema.

In a gateway like the VTEP L2 gateway, physical vlans belonging to
the same logical network form a "logical switch".  Each logical switch
has a dedicated tunnel key and will keep records of all MACs learned
from the owned vlans.  So user can just send packet to a "logical
switch" and the gateway will figure out the output port and vlan tag
automatically.

Therefore, it is really not necessary to keep record of the vlan map
for each gateway physical port in the OVN_Southbound database using
"gateway_ports" and to map each vlan to a unique ovn logical port.
Instead, we should simply map each logical switch to a ovn logical
port.

Thusly, this commit removes the "Gateway" table from the OVN_Southbound
database.  In the "Chassis" table, the "gateway_ports" column is replaced
by "vtep_logical_switches" column which stores all vtep logical switch
names.  The use of this column will be documented in later commit.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Russell Bryant <rbryant@redhat.com>