Allow deploy-exclude-additions.txt to be defined

[grasmash]

In addition to allowing deploy-exclude.txt to be replaced with a custom file, we should allow an additional deploy-exclude-additions.txt file to be defined. This will allow projects to add additional lines to be excluded from deployment artifacts without the need to frequently rebase from upstream.

• Add new deploy.exclude_additions_file property to build.yml with default value ${repo.root}/blt/deploy-exclude-additions.txt
• Create empty template/blt/deploy-exclude-additions.txt file.
• Document new setting here
• Modify deploy:copy target such that it:
  • Checks for the existence of ${deploy.exclude_additions_file}
  • If it exists, munge ${deploy.exclude_additions_file} and ${deploy.exclude_file} together into a third temporary file.
  • Use the temporary file in the rsync command