Wireup sequences upgrade step

Add the upgrade step to add EnvUUID to sequence docs to the 121
upgrade steps.
state: added State.EnvironUUID and use it everywhere

EnvironTag().Id() is commonly used to get the environment UUID. A
EnvironUUID method has been added to provide a more direct and easier
to read way of obtaining the environment UUID.
Merge pull request #1009 from waigani/wireup-sequences-step

Wireup sequences upgrade step

Add the upgrade step to add EnvUUID to sequence docs to the 121
upgrade steps.
Merge pull request #1007 from mjs/envuuid-method

state: added State.EnvironUUID and use it everywhere

EnvironTag().Id() is commonly used to get the environment UUID. A EnvironUUID method has been added to provide a more direct and easier to read way of obtaining the environment UUID.
Merge branch 'master' of https://github.com/juju/juju into migrate-charms