State upgrade step: Add EnvUUID to minUnits docs.

Add an upgrade step to add the environment uuid to minUnits docs. Update doc struct, call sites and tests.
Merge branch 'master' of https://github.com/juju/juju into migrate-minUnits

Conflicts:
	state/upgrades_test.go
	upgrades/steps121.go
	upgrades/steps121_test.go
Merge branch 'master' of https://github.com/juju/juju into migrate-minUnits

Conflicts:
	state/upgrades.go
	upgrades/steps121.go
Merge branch 'master' of https://github.com/juju/juju into migrate-minUnits

Conflicts:
	upgrades/steps121.go
Merge pull request #983 from waigani/migrate-minUnits

State upgrade step: Add EnvUUID to minUnits docs.

Add an upgrade step to add the environment uuid to minUnits docs. Update doc struct, call sites and tests.