sbr: Add rule for outbound interface when there is a single interface IP

[neilcook]

When using the sbr plugin with a CNI plugin such as multus, applications have to bind to the source IP address of the interface for the str plugin to work correctly. They cannot bind to the interface name, because the sbr plugin doesn't currently add a rule for the interface name.

This is problematic because it is straightforward to configure the name of a multus interface, however the IP address is usually assigned from a range, so the exact interface IP address to use is not known in advance unlike the interface name. It would thus be much easier for applications if the sbr plugin added a rule with the interface name in addition to the rule with the interface address.

This PR does exactly that - adds an additional rule with the outbound interface name, so that applications can configure the name of the interface to bind to, rather than having to know the interface IP address.

Obviously, this approach does not work if there are multiple IP addresses for the interface, so this change only adds the rule for the outbound interface name if there is a single IP address on that interface.

I have added tests for the new rule, which all pass.