fix: Check if example directory already exists

[TannerGabriel]

Currently, when the multistage-delivery.sh script of the quickstarter example is run multiple times, it fails since the examples repository already exists.

fatal: destination path 'examples' already exists and is not an empty directory.

Now the existence of the directory gets checked before cloning the repository to remove the possibility of this error.

Fixes

Keptn.sh issue

[TannerGabriel]

Another option would be to ask the user if he wants to replace the directory.

read -p "Examples directory already exists. Want to overwrite it (y/n)?" choice
case "$choice" in 
  y|Y ) echo "Replace directory";;
  n|N ) echo "Keep old directory";;
  * ) echo "invalid";;
esac