Re-arrange MAC election code for sandbox iface

- in bridge.go

Signed-off-by: Alessandro Boch <aboch@docker.com>

drivers/bridge/bridge.go

@@ -404,12 +404,8 @@ func (d *driver) CreateEndpoint(nid, eid types.UUID, epOptions map[string]interf
 		}
 	}()
 
-	mac := netutils.GenerateRandomMAC()
-	// Add user specified attributes
-	if epConfig != nil && epConfig.MacAddress != nil {
-		mac = epConfig.MacAddress
-	}
-
+	// Set the sbox's MAC. If specified, use the one configured by user, otherwise use a random one
+	mac := electMacAddress(epConfig)
 	err = netlink.LinkSetHardwareAddr(sbox, mac)
 	if err != nil {
 		return nil, err
@@ -613,6 +609,13 @@ func parseContainerOptions(cOptions interface{}) (*ContainerConfiguration, error
 	}
 }
 
+func electMacAddress(epConfig *EndpointConfiguration) net.HardwareAddr {
+	if epConfig != nil && epConfig.MacAddress != nil {
+		return epConfig.MacAddress
+	}
+	return netutils.GenerateRandomMAC()
+}
+
 // Generates a name to be used for a virtual ethernet
 // interface. The name is constructed by 'veth' appended
 // by a randomly generated hex value. (example: veth0f60e2c)