Rename 'history' -> 'Release notes'
Rename 'history' -> 'Release notes'

This seems to better match what the other oslo
libraries are calling this section, so we might as
well call it that to.

Change-Id: I67773ef2241dcf2f2a1cd65756ac0f8a8081c9cb

pombredanne/taskflow-1,jimbobhickville/taskflow,openstack/taskflow,jimbobhickville/taskflow,pombredanne/taskflow-1,openstack/taskflow