Document tools/flakes is from commons

When copying tools around it may not be clear that they came from
openstack-common.  So add a note at top of code. Had to say from
openstack-common since cannot say from oslo-incubator since word oslo is
replaced when copied.

Change-Id: I7514c88d448f13cb7b11ed1ea81a035c6a201bae
Merge "Document tools/flakes is from commons"