ovn-nbctl: Make daemon mode more transparent.

ovn-nbctl: Make daemon mode more transparent.

This makes ovn-nbctl transparently use daemon mode if an appropriate
environment variable is set.

It also transforms ovn-nbctl.at so that it runs each ovn-nbctl test in
"direct" mode and in daemon mode.  It uses a combination of m4 macros and
shell functions to keep from expanding the generated testsuite more than
necessary.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Mark Michelson <mmichels@redhat.com>