labs: always init local variable
labs: always init local variable

Local variable LINE was not initialized (but still used in a log call)
if disk_registered returned false. Fixed.

Change-Id: Icd679b2e50fd0d11fb5689d5806a016ac8043f7f

openstack/training-guides,harrisonfeng/training-guides,Nabs007/training-guides,harrisonfeng/training-guides,harrisonfeng/training-guides,openstack/training-guides,Nabs007/training-guides,Nabs007/training-guides