Add a juju action for upgrading the schema

[Free Ekanayaka]

We should add a ./actions/migrate-schema script and associated actions.yaml metadata with code that will essentially call "schema --upgrade" in order to perform schema upgrades/migrations.

Initially it should be a "migrate and hope for the best" action, but in the future (in bug https://bugs.launchpad.net/landscape-charm/+bug/1456953), we should check for the number of connected landscape users first.

Initially the script can be invoked via juju run, since Juju actions are not yet officially supported.

[Данило Шеган (danilo)]

FWIW, "schema" script does not have an "--upgrade" argument, but if called without any arguments, it will do a couple of things which end up being noop when schema is there, and then upgrade the schema. So, I believe that calling "./schema" should be sufficient for the "migrate-schema" action.