Use mtu setting from table instead of flag

Adds mtu to argument lists of key methods and configuration of vlan
interface. Also adds deprecation notice to the "help" info for the file
based network_device_mtu configuration property.

Partially-implements blueprint better-support-for-multiple-networks

Change-Id: Ia401eb95ce8c81d104e59b0348e3474655b9a992
Merge "Use mtu setting from table instead of flag"