State, upgrade step: Add EnvUUID to Sequence doc IDs

Adds an upgrade step to add the EnvUUID to sequence doc IDs.
generate-tools now merges metadata from different streams
Review fixes
Merge branch 'master' of https://github.com/juju/juju into migrate-sequences
Merge branch 'master' of https://github.com/juju/juju into migrate-sequences
Merged 1.21 upgrade steps into one list

Without this, upgrade steps really end up running in the wrong
order. Steps like the machine and instanceData env UUID migrations
really need to go first.

This change solves the immediate issue. A future change will tidy
this up so that the steps can be assigned against 1.21 final instead
of alpha1.
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.
Merge pull request #1008 from mjs/merge-121-upgrade-steps

Merged 1.21 upgrade steps into one list

Without this, upgrade steps really end up running in the wrong order. Steps like the machine and instanceData env UUID migrations really need to go first.

This change solves the immediate issue. A future change will tidy this up so that the steps can be assigned against 1.21 final instead of alpha1.
Small tweak
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.
Small tweak
Introduce constants
Do not read entire charm into memory when downloading
Small tweaks
Merge pull request #1001 from wallyworld/simplestreams-merge

generate-tools now merges metadata from different streams

Fixes: https://bugs.launchpad.net/juju-core/+bug/1386509

juju metadata generate-tools creates simplestreams metadata for tools tarballs.
When run more than once, it now appends any new metadata for a new stream to the existing metadata.
Merge pull request #1012 from wallyworld/local-charm-memory

Do not read entire charm into memory when downloading

Fixes: https://bugs.launchpad.net/bugs/1386405

Do not pull entire charm into memory when downloading.
cmd/juju: update docs with info on placement directives

Also, bring add-machine docs up-to-date and a bit more cohesive.

Fixes https://bugs.launchpad.net/juju-core/+bug/1387421