Trigger SELinux autorelabel on first boot.
Trigger SELinux autorelabel on first boot.

This adds about 30 seconds to my local boot time.

Fixes bug #1179730

Change-Id: I519bb9289236abd43f8eb784768dcab10e2e5754

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