refactor Terraform code into modules

[afeld]

Closes #6.

This pull request refactors the Terraform code into a few modules:

• instances, which is likely what we'll use to add to an existing security group in D2D
• networking, which creates (at the moment) the security group for the Jenkins master
• standalone, which is relevant if the user wants to set up Jenkins with its own security group
• The "root" module (which already existed), which is used for integration testing of this repository

[afeld]

Also, I am very open to alternate ideas on module names.

[afeld]

Come to think of it, is there any downside to having a separate security group created for Jenkins specifically, versus sharing an existing one?