Fix several issues in BUILDING.md, add missing config options

[ghost]

Issue #, if available:

Description of changes:

• Add deployer config options to correspond to the rest of the SAM template.
• Move the deployer config docs from the example deployer script to BUILDING.md in the root.
• Fully document the deployer config file, including the newly added options.
• Link to the example deployer config scripts from the relevant step in BUILDING.md.
• Document the test scripts.
• Recast the deployment config parsing so it's easier to understand. (This helped me tremendously in creating this patch.)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.