Introduce fast-forward upgrades
Introduce fast-forward upgrades

Fast-forward upgrades are upgrades that move an environment from release
`N` to `N+X` in a single step, where `X` is greater than `1` and  for
fast forward upgrades is typically `3`. This spec outlines how such
upgrades can be orchestrated by TripleO between the Newton and Queens
releases.

Change-Id: I7a422f2355b63dde788207d6ff388f6bee594b33

openstack/tripleo-specs,openstack/tripleo-specs