Mac/Linux quickstart cleanup.

[bcipriano]

Some minor cleanup in the quickstarts.

• Use the new script get-latest-release-tag.sh added by https://github.com/AcademySoftwareFoundation/OpenCue/pull/1244. This ensures the guides always point to the latest release, rather than needing an update every time we cut a new release.
• Rename the virtual environment sandbox-venv. This adds clarity and avoids conflicts with any existing venv that may exist.
• For the Linux version:
  • Switch to Python 3, it is preferred in all possible cases over 2 now.
  • Remove steps setting OL_CONFIG and CUEBOT_HOSTS. Since this guide was written the code has been updated to use default values that make these steps redundant.

[bcipriano]

Merging this TBR: important to get these quickstarts updated ASAP as the current versions are now using a very old release.