Switch license to CC-BY 3.0.

I asked on the legal-discuss mailing list which license should be used
and the direction given was to use CC-BY for new documentation.

http://lists.openstack.org/pipermail/legal-discuss/2014-March/000201.html

Change-Id: I24a5f0e6de048ef643f42dc7efe5e4cdc35772ba