Note: This addresses a bug with MacOS introduced with c73f6149a92d0b1…

…c3d909381e33c4cf2e718e964 because "read -i" requires Bash >= 4 (newer than 2009 !) as OSX only comes with Bash 3.2

Refactor of Prompt function has:

  - editable prompts with read -i ONLY if Bash >= 4
  - in cases where you don not want or need an editable prompt (e.g. AMI ID) you leave out the 3rd argument
  - fully compatible with previous prompt function (only cosmetic changes in azure template)