net: bridge plugin should allow bridging a host iface

[eyakubovich]

As a convenience feature, bridge plugin should have an ability to connect host interface to the bridge (when the bridge is created).

[jonboulle]

@eyakubovich move this to CNI?

[steveej]

Is the idea to transfer all the attributes of the given interface to the bridge and then enslave the interface?

I think the approach is potentially dangerous because the host can lose its network connectivity with the given interface at least for a short period of time.