vswitchd: Configuration of allowed OpenFlow versions

vswitchd: Configuration of allowed OpenFlow versions

Versions may be configured using the protocols column of
the bridge table. The protocols column is a set which accepts zero
or more of the values: 'OpenFlow10' and 'OpenFlow12'.

If the protocols column is empty, then OpenFlow10 is used.
This default is consistent with the behaviour of ovs-vswtichd
prior to this patch.

Signed-off-by: Simon Horman <horms@verge.net.au>
[blp@nicira.com adjusted comments and documentation]
Signed-off-by: Ben Pfaff <blp@nicira.com>