ovs-vsctl: Create AutoAttach records on demand, for backward compatibility.

ovs-vsctl: Create AutoAttach records on demand, for backward compatibility.

The AutoAttach table is new in OVS 2.3.90, but ovs-vsctl was creating a
record in the table unconditionally whenever it created a new bridge.
This caused a gratuitous incompatibility with older databases, which can
be a problem during upgrade.  This commit switches to creating the
AutoAttaach record for a bridge lazily, only when the first mapping is
created for the bridge.

VMware-BZ: #1413562
CC: Dennis Flynn <drflynn@avaya.com>
Reported-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>