Always match lowercase MAC

In some places we are making comparisons between MACs
from different sources like:
* from DB - those are lowercase strings
* from other inputs - where we do not know if MAC will
  be lowercase, uppercase (or mixed) string
We need to always lowercase string in DB queries.

Also new utils function was created: is_same_mac to compare
if MACs are equal. It uses netaddr.EUI internally.

Change-Id: Idae5bc6009f857e72712afac04ffe174e73c7a87
Closes-Bug: #1394466
(cherry picked from commit dfe79d35a47a28873f271131cf2fdae7052e0713)