Remove duplicate file extensions.
Remove duplicate file extensions.

Gracefully remove duplicate file extensions on the output filename

Change-Id: Id4f8b0d75b587c238c89d6ca7a06fab6a57cd627

switch-ch/diskimage-builder,agroup/tripleo-puppet-elements,takahashinobuyuki/diskimage-builder,qwefi/tripleo-image-elements,qwefi/tripleo-image-elements,agroup/tripleo-puppet-elements,openstack/diskimage-builder,agroup/tripleo-puppet-elements,citrix-openstack-build/diskimage-builder,rdo-management/diskimage-builder,qwefi/tripleo-image-elements,varunarya10/diskimage-builder,openstack/tripleo-image-elements,openstack/diskimage-builder,switch-ch/diskimage-builder,varunarya10/diskimage-builder,JioCloud/diskimage-builder,zioproto/diskimage-builder,rdo-management/tripleo-image-elements,citrix-openstack-build/diskimage-builder,radez/tripleo-image-elements,qwefi/tripleo-image-elements,NaohiroTamura/diskimage-builder,JioCloud/diskimage-builder,rdo-management/diskimage-builder,zioproto/diskimage-builder,takahashinobuyuki/diskimage-builder,agroup/tripleo-puppet-elements,qwefi/tripleo-image-elements,CSC-IT-Center-for-Science/diskimage-builder,openstack/tripleo-image-elements,rdo-management/tripleo-image-elements,eantyshev/diskimage-builder,radez/tripleo-image-elements,eantyshev/diskimage-builder,NaohiroTamura/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder