charm-build should be able to rebuild from a build-manifest.json

[johnsca]

After #403 and #404 done, it would be ideal to be able to pass a build-manifest.json file in to charm-build so that an exact duplicate, save for the top-level charm layer changes, of that charm can be built. This is particularly important if a charm author wants to create a small bug fix for a charm but finds that many of the base layers have changed.

[ajkavanagh]

Please see my comment on #403 re: reproducible builds for stable releases. I suspect it is more relevant here, so I'll copy the link: build-dependencies.md