[WIP] Support for filtering status on different criteria.

- From spec: https://docs.google.com/a/canonical.com/document/d/1olIjfnMIKPbNMmNBhfjwlnzdRvY-xBKygpIOhv7QIkE/edit#heading=h.tdsd6gcvc55e
Merge, refactoring, additional filters, tests.

- Merge branch 'master' into status-filtering.
- Filters moved into their own file.
- Now filtering machines as first-class objects.
- Filters broken down into atomic operations for re-usability.
- Meat of tests is now present.
Merge remote-tracking branch 'refs/remotes/origin/master' into status-filtering
Migration code for JobManageNetworking added, tests still missing.
Holistic unit-chain matching.

- Situations in which the entire chain of units and their subordinates do not match are now handled correctly.
Switching local updates to default of true.

We don't want local installs of Juju to become outdated, so unless the user has specified otherwise, we will default this value to true.
Refactor peergrouper tests to avoid cycle imports

The tests for peergrouper, being part of peergrouper package
would trigger an import cycle if anything under apiserver
imports peergrouper.
The tests where refactored to use the export_test pattern
so all tests are under peergrouper_test package
Added functionality for stubbing embedded machines.
Merge remote-tracking branch 'refs/remotes/origin/master' into status-filtering
Go formatted
Merge branch 'uniter-context-factory' into uniter-context-factory-2
Possible fix for Bug #1304126

In the SetEnvironmentCommand's Init function I added an additional API call to retrieve existing environment config.
All keys a user specifies in the "juju set-env" command are checked against the existing config keys and if the
user defined key does not exist in the existing config a warning of a possible misspelling is issued.
Fixed capitalization in some of the returned error messages.
Restructured the code so that in the unlikely event that c.NewAPIClient() returns an error, the following code will not fail/panic/etc.
Fix style-guide links

Changed links to absolute, was improperly linking to /doc/contributions/doc/contributiuons/style-guide.md
Increment juju to 1.21-alpha3
- Moved extra API calls to the Run function.
- Added a similar check for "juju unset-env", which checks if the key the user is trying unset exists in the config
  and displays a warning if it does not.
Added tests for the migration of JobManageNetworking.
Merge branch 'master' of https://github.com/juju/juju into job-manage-networking-upgrade
A proposed fix for bug #1384622

Added a check to the "juju set" command, which issues a warning to the user
if the value of the configuration option is already set to the desired value.

The check is done by retrieving the unit's configuration options via an
additional API call and comparing the user-defined value with the
existing configuration value.