ovn-nbctl: Enable database commands using db-ctl-base infrastructure.

ovn-nbctl: Enable database commands using db-ctl-base infrastructure.

This makes ovn-nbctl into a pretty slavish imitation of ovn-sbctl, using
almost the same code.  It has two immediate benefits.  First, multiple
commands can now be chained together into a single ovn-nbctl invocation.
Second, the database commands such as "create", "set", and so on allow
queries and modifications that don't have specific commands already.
In the following commit, this allows testing the OVN ACL feature.

Signed-off-by: Ben Pfaff <blp@nicira.com>