db-ctl-base: Allow print rows that weak reference to table in

db-ctl-base: Allow print rows that weak reference to table in
'cmd_show_table'.

Sometimes, it is desirable to print the table with weak reference to
the table specified in 'struct cmd_show_table'.  For example the
Port_Binding table rows in OVN_Southbound database that refer to the
same Chassis table row can be printed under the same chassis entry
in 'ovn-sbctl show' output.

To achieve it, this commit adds a new struct in 'struct cmd_show_table'
that allows users to print a table with weak reference to 'table'
specified in 'struct cmd_show_table'.  The 'ovn-sbctl' which now prints
the Port_Binding entries with Chassis table, is the first user of this
new feature.

Requested-by: Justin Pettit <jpettit@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>