Add AZUL_S3_BUCKET environment variable to `install_bootstrap.sh` and make changes to mustache template.

[mjkrause]

This PR is part of a series of changes to implement the functionality to upload a BDBag to an S3 storage location. Specifically I

• added a question to install_bootstrap that asks the user for the name of the S3 bucket as part of the installation process
• added an environment variable named AZUL_S3_BUCKET to install_bootstrap

Then I added the azul_s3_bucket variable to

• dev.yml and prod.yml in the Boardwalk directory
• the mustache config file in boardwalk_launcher_config/boardwalk.config

I successfully performed an installation.

[coverbeck]

Your commit message should ideally reference the GitHub issue. Unfortunately the JIRA ticket was created in the TLP project; should have been created in DEP. Can you move the ticket to DEP, and that may create the corresponding issue in this repo?

If it doesn't then you can still refer to the GitHub issue in the other repo.

There is GitHub markdown for mentioning issues.