Test MAC address should be 6 octets.

Since nova commit Idcba745a011604d7664b62d88f1dede88d10443d, the nova
api now validates MAC addresses.  The test MAC address we were using in
register-nodes was not valid.

Change-Id: I0e60a20e503098532b81c5ee7942e99f0180a4f9
Merge "Test MAC address should be 6 octets."