Clean up L2ForwardingService

Clean up L2ForwardingService

Avoid multiple instanciations of MatchBuilders.

Factor common code involved in FlowBuilder setup and instruction
construction, adding appropriate methods in FlowUtils and
InstructionUtils.

Use fluent-style builders.

Drop appendResubmitLocalFlood() which isn't used.

Avoid multiple creations of the same Uri in loops.

Two of the FlowBuilders don't have a setStrict() call; is that
intentional?

Change-Id: I984a735229613bd013443ff16815a1566a74b2d5
Signed-off-by: Stephen Kitt <skitt@redhat.com>