Expand cloudformation provisioning to allow override of defaults

[jstuever]

Expanding the provisioning/cloudformation playbooks to allow overriding of key parameters.

• cloud_formation template can now be overridden, ie. to a custom template in the meta repo
• security group ids can now be overridden.
• remove unnecessary template parameter from cloudformation_teardown

[detiber]

@jstuever let's discuss this more next week (I'm at All Things Open the next couple of days). I've started a PR to take this a bit different way: #41

[jstuever]

Closing in favor of #41