Make it clear that tmpfs is optional
Make it clear that tmpfs is optional

Given this is often the final output, it can look like an error occured.
Changing the wording makes this clearer.

Change-Id: I70f157054e3120cffee6fa5241b1ffe0b7bfa650

switch-ch/diskimage-builder,openstack/diskimage-builder,takahashinobuyuki/diskimage-builder,takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,rdo-management/diskimage-builder,openstack/diskimage-builder,rdo-management/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,zioproto/diskimage-builder,switch-ch/diskimage-builder