Add spec for online-schema-upgrades
Add spec for online-schema-upgrades

This commit adds spec explaining how to implement DB schema upgrades
in a live way to achieve ability to do rolling (or non-downtime)
upgrades of Cinder.

Change-Id: I19b1128335cc26fbf3287710a4cb974f695c89ba
Implements: bp online-schema-upgrades

openstack/cinder-specs