ovs-ofctl: Add option to set allowed OpenFlow versions

ovs-ofctl: Add option to set allowed OpenFlow versions

--protocols allows configuration of the versions
that may be used when establishing an OpenFlow connection.

The default is 'OpenFlow10' which is consistent with
the behaviour prior to this patch.

The useful values at this time are:
'OpenFlow10', 'OpenFlow12', 'OpenFlow13',
Values may be combined in a comma delimited list.

e.g.: --protocols 'OpenFlow10,OpenFlow12,OpenFlow13'

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>