Use the external MgoSuite.

Mostly mechanical.
Remove the testing/mgo* bits which are now in github.com/juju/testing.
Add testing.Certs to be used when registering the MgoSuite tests.
store: use IsolationSuite in ConfigSuite
Add testing package tests, rename getCerts.
Merge pull request #62 from rogpeppe/008-store-configsuite-isolationsuite

store: use IsolationSuite in ConfigSuite

This removes a dependency on github.com/juju/juju/testing.
Merge pull request #61 from frankban/external-mgosuite

Use the external MgoSuite.

Mostly mechanical.
Remove the testing/mgo* bits which are now in github.com/juju/testing.
Add testing.Certs to be used when registering the MgoSuite tests.