tests: Set explicit bond mode in LACP test.

tests: Set explicit bond mode in LACP test.

This avoids a log warning:

    bridge|WARN|port bond: Using the default bond_mode active-backup.
    Note that in previous versions, the default bond_mode was balance-slb

This warning is harmless, but I'm trying to add checks for "warn" and
higher severity log messages to the tests, so it makes sense to get rid of
this one.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>