simplebridge: only create network/<network>/bridgeInterface once, also

... fix the tests
So we can re-use the range, notice that we never delete from the map either. It’s just replacement.

> On Dec 7, 2014, at 5:43 PM, Alexander Morozov <notifications@github.com> wrote:
> 
> In extensions/simplebridge/ipallocator.go:
> 
> > +	)
> > +
> > +	ipMap, err := ip.Refresh()
> > +	if err != nil {
> > +		return nil, err
> > +	}
> > +
> > +	lastip := ip.bridgeNet.IP
> > +
> > +	for {
> > +		rawip := ipToBigInt(lastip)
> > +
> > +		rawip.Add(rawip, big.NewInt(1))
> > +		newip = bigIntToIP(rawip)
> > +
> > +		if !ip.bridgeNet.Contains(newip) {
> I'm not fully understand this logic. Is this means that you try every IP in network twice? Why not exit if you out of range immediately?
> 
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