Redo PGP, service state, and defaults location

- Change default location of the stackdriver template for Debian based systems. Should be `…/etc/default/stackdriver-agent.erb` as opposed to `…/etc/default/stackdriver.erb`

- Modify apt key logic to grab the package signing key directly from Stackdriver, rather than PGP (PGP's subkey server was not responding during testing).

- Add logic to enable/disable the stackdriver service. 

Potentially any of these commits could be cherry picked, but the first two were necessary to get this working on Ubuntu 12.04 (Precise) in AWS.
Duplicated these variables on `init` in order to pass them through to the `stackdriver::service` class.