[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
Holistic unit-chain matching.

- Situations in which the entire chain of units and their subordinates do not match are now handled correctly.
roughly break up test files; extract context.Factory
add a bunch of failing tests
add basic tests for Factory; move non-Factory NewHookContext func into export_test (to keep existing tests passing, but prevent other people from creating contexts in unhelpful ways
remove persistent relation context from Relationer
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We don't want local installs of Juju to become outdated, so unless the user has specified otherwise, we will default this value to true.
Added functionality for stubbing embedded machines.
Merge remote-tracking branch 'refs/remotes/origin/master' into status-filtering
address review
drop proxy rubbish
Merge branch 'uniter-context-factory' into uniter-context-factory-2
Fix style-guide links

Changed links to absolute, was improperly linking to /doc/contributions/doc/contributiuons/style-guide.md
Mostly working set. Services are not being filtered.
provider/ec2: verify credentials early in bootstrap

Verify credentials early in the bootstrap process by
issuing a query to EC2 and checking the status code
on the response. Provide a user-friendly error message,
and log the underlying error at debug level.

Partially fixes https://bugs.launchpad.net/juju-core/+bug/1362072
provider/azure: use new in-memory cert/key API in gwacl

A third-party contribution has just landed in gwacl to
allow users to pass the certificate and key as a []byte,
rather than requiring filenames to be specified. Juju
is updated to use this, removing the need to create
temporary files, simplifying the code.
Add comments about immutability/snapshots
utils/ssh: fix flaky run tests

Timeout should be long if the operation
is not expected to timeout.