State, upgrade step: Add EnvUUID to Sequence doc IDs

Adds an upgrade step to add the EnvUUID to sequence doc IDs.
Merge branch 'master' of https://github.com/juju/juju into migrate-sequences
Merge branch 'master' of https://github.com/juju/juju into migrate-sequences
Merge pull request #973 from waigani/migrate-sequences

State, upgrade step: Add EnvUUID to Sequence doc IDs

Adds an upgrade step to add the EnvUUID to sequence doc IDs.