Use the charmcraft reactive plugin instead of using overrides (also unlock charmcraft pin at 1.5/stable)

[ajkavanagh]

At present, the charmcraft.yaml file for reactive charms uses a set of stage/part overrides to complete the build rather than using the reactive plugin. This, due to some env variable name changes has meant that we are pinned to 1.5/stable (of charmcraft).

It would be better to use the reactive plugin and enhance that with patches so that we can unpin 1.5/stable and also have a more resilient charmcraft build going forwards. Also see: #217 comment on global/source-zaza/charmcraft-build-lock-file.yaml. @freyes