Generate local mac address for the bridge

Fix #3200. (require kernel v3.3 or later)

By default, the bridge attaches to the lowest mac address of the
interfaces it enslaves. However, the interfaces are added or deleted
as containers start or stop. So the bridge changes its mac address
from time to time, and causes the network disrupted temporarially.

Kernel 3.3+ and later allows the bridge to have its own mac address.
Once the bridge has its own mac address, it will stick with that one.

Signed-off-by: Tzu-Jung Lee <roylee17@gmail.com>