Fix syntax error in GRUB_OPTS env var handling
Fix syntax error in GRUB_OPTS env var handling

Reference to ${GRUB_OPTS:""} in 'vm' element causes syntax error
if GRUB_OPTS is set. Fix by changing to ${GRUB_OPTS:-""}.

Change-Id: Iae3ee8b7c4064c92d0cfc1b5a8caa22e53d6d112

varunarya10/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,JioCloud/diskimage-builder,zioproto/diskimage-builder,openstack/diskimage-builder,JioCloud/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,rdo-management/diskimage-builder,switch-ch/diskimage-builder,NaohiroTamura/diskimage-builder,eantyshev/diskimage-builder,takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,rdo-management/diskimage-builder,citrix-openstack-build/diskimage-builder,zioproto/diskimage-builder,NaohiroTamura/diskimage-builder,citrix-openstack-build/diskimage-builder,eantyshev/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,varunarya10/diskimage-builder