debootstrap: fix syntax issues
debootstrap: fix syntax issues

There's no need for 'break' to exit a switch case, ';;' will work fine.

Change-Id: I4f23c80422a3c1dbd0db3972854ae67e38fc7be8

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