Add Upgrade impact section to the spec template
Add Upgrade impact section to the spec template

It happens that we overlook the upgrade impact of some new
features. Add Upgrade impact section to the spec template to make us
think about how do we upgrade to some feature when proposing that
feature.

Change-Id: I137a4353ce73ae961e83ecd656df77d6bd45a4e4

openstack/tripleo-specs,openstack/tripleo-specs