Don't restart mongo every time jujud starts.

Add a public method to let us know if the service exists as expected.
Check to see if the service already exists before writing it out again.
Refactor the upstart exists to use once source, and fix the tests so it doesn't create a braindead empty upstart file first.
Start the service if we need to.
Merge pull request #424 from howbazaar/fix-juju-db-upstart-rewrites

Fix juju db upstart rewrites

Export a method from the Service types - Exists.

Use this in the mongo ensure method.