The ip_lib code is able to get the MAC address of a device
already using the IpLinkCommand class, let's remove
get_interface_mac() since it's not necessary.
Changed all callers to use the new get_device_mac() code
and tagged get_interface_mac() for deprecation since it
could be in use by other callers outside the neutron tree.
Closes-Bug: #1630439
Change-Id: I1695d7e46efe5245eb581bd40d5420250a3bad89