Deal with uppercase MAC address in register-nodes

Ironic will downcase all MAC addresses that are registered, so if
the node description uses uppercase MAC addresses, they will not
be matched. Lowercase when checking MACs against the node mapping
to sort this out.

Change-Id: I322e5d9eeb3ed85dd42d67caff8c61cea8c23856
Closes-Bug: #1394084
Merge "Deal with uppercase MAC address in register-nodes"