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.