n8g-sfc currently has its own variant of OVSBridge to allow the use
of priority in a delete_flows call
This change is meant to make this available outside n8g-sfc and
simplify n8g-sfc code.
This change adds a 'strict' boolean parameter to mod_flow and delete_flows
that results in ovs-ofctl to be run with --strict for del-flows and
mod-flows actions. When strict is set, the use of priority is allowed
and hence not rejected anymore.
Note that for batched actions in a deffered bridge, we disallow mixing
calls with strict and without strict, which can't be translated in one CLI