After recent conversations, we've come to the conclusion it will be
good to have two models of promotion
- using tags, where gate directly uploads to the final repository and
promote retags the image.
- from an intermediate-registry, where upload stores the built image
in an i-r and the promote step uploads to the final registry.
To facilitate this, we add a "promote_container_image_method" flag to
the promote roles.
The documentation is expanded to explain how all this is intended to
work together.
These roles haven't been publicised yet, but this should be a no-op as
it defaults to tags, which is the current operation.
c.f. Ia24bbd101e01ab371ceacfed006b5ff806418a97
Change-Id: I1c25f60f835b1cab983bcdd169eeffc0e250a56c
0a64d51c3d