Add support for the -R flag on macOS.

[pjcreath]

What does this PR do?

Allows macOS to bootstrap from alternate repos.

What issues does this PR fix or reference?

Fixes #1870

Previous Behavior

sudo sh bootstrap-salt.sh -R archive.repo.saltproject.io -D stable 3004

 *  INFO: Running install_macosx_stable_deps()
 *  INFO: Running install_macosx_stable()
 * ERROR: https://repo.saltproject.io/osx/salt-3004-py3-x86_64.pkg failed to download to /tmp/salt-3004-py3-x86_64.pkg
 * ERROR: Failed to run install_macosx_stable()!!!

New Behavior

sudo sh bootstrap-salt.sh -R archive.repo.saltproject.io -D stable 3004

 *  INFO: Running install_macosx_stable_deps()
 *  INFO: Running install_macosx_stable()
installer: Package name is Salt 3004 (Python 3)
installer: Upgrading at base path /
installer: The upgrade was successful.