bridge: Store datapath version into ovsdb

OVS userspace are backward compatible with older Linux kernel modules.
However, not having the most up-to-date datapath kernel modules can
some times lead to user confusion. Storing the datapath version in
OVSDB allows management software to check and optionally provide
notifications to users.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>