ovsdb: Add unixctl commands for OVSDB replication

ovsdb: Add unixctl commands for OVSDB replication

Set and get the server to replicate from:

    ovsdb-server/set-remote-ovsdb-server {server}
    ovsdb-server/get-remote-ovsdb-server

Set and get the replicated table blacklist:

    ovsdb-server/set-sync-excluded-tables {DB:table,...}
    ovsdb-server/get-sync-excluded-tables

Connect to the configured server and start replication:

    ovsdb-server/connect-remote-ovsdb-server

Disconnect from the remote server and stop replication, without dropping
the replicated data:

    ovsdb-server/disconnect-remote-ovsdb-server

Signed-off-by: Mario Cabrera <mario.cabrera@hpe.com>
Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Andy Zhou <azhou@ovn.org>