Update specs template
Update specs template

Partially merge changes to the specs template from Rocky nova-specs.

Still leaving out stuff that shouldn't apply to nova-powervm, like
notifications and REST API changes.

Change-Id: If2df2b278ba555cf4d5d04cca19079c2749ca202

stackforge/nova-powervm,openstack/nova-powervm,openstack/nova-powervm,stackforge/nova-powervm