API: Validate and normalize address
API: Validate and normalize address

On the API side, checks whether the supplied MAC address is formally
correct and normalize it to all lower case.

The patch also improves the docstring of the is_valid_mac function.

Change-Id: I6ae13888a9e17a10bba20acf829cbd01df535a3c
Closes-Bug: #1259269

openstack/ironic-lib,NaohiroTamura/ironic-lib,citrix-openstack-build/ironic-lib,faizan-barmawer/elytics,faizan-barmawer/ironic-lib