Fix various issues in live tests, dummy provider

MgoSuite was missing from dummy provider. Various
tests failed to bootstrap correctly, relying on
previous tests to do so; various tests were not
releasing resources properly, leaking mgo sessions.
provider/ec2: Fix fallout from live test changes
juju: add missing MgoSuite
juju/testing: keep tearing down if closing state fails

mongod processes were being left around because of
failures to close State objects. We fix this by
changing the Assert to Check.
Add reviewing checklist to docs/contributions. There is some overlap with the style guide. Where the style guide aims to be descriptive, this document is simply a list of questions for the reviewer to ask when when reading a PR.
Merge pull request #457 from axw/fix-tests-mgo-leakage

Fix tests mgo leakage

Various fixes to tests, mostly relating to usage of MgoSuite usage.
Merge branch 'master' into review-doc
Merge pull request #441 from waigani/review-doc

Add reviewing checklist to docs/contributions.

There is some overlap with the style guide. Where the style guide aims to be descriptive, this document is simply a list of questions for the reviewer to ask when when reading a PR.