Local shell

[umputun]

This PR adds a new top-level field to playbooks (both full and simplified versions), allowing to set the local shell (i.e. local_shell=/bin/bash)

This is because there are unpredictable and unexpected results if we run some scripts with local mode on a different host. For example, this simple script works fine with #/bin/sh but doesn't work correctly with #/bin/zsh

  - name: "make images"
    script: |
     IMAGES="website email-sender private-share"
      mkdir -p /tmp/web-images
      for image in ${IMAGES}; do
        echo "process image: ${image}"
        docker pull  example.com/miscs/${image}:latest
        docker save "example.com/miscs/${image}" -o "/tmp/web-images/${image}.tar"
      done
    options: {local: true}

The change is back compatible, i.e., if local_shell is not set, it will follow the old logic (os.Getenv("SHELL") with failback to /bin/sh)