Allow multinode test to proceed if no loop device
Allow multinode test to proceed if no loop device

Rather than skipping the entire test, skip the validation step
and let the rest of the function complete if no loop devices
are found.

Change-Id: Iff281e861601ce198e7a865c19180c121d329a97

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server