Dynamic targets

[umputun]

This PR adds support for dynamic targets in some limited use cases.

tasks:
  - name: get host
    targets: ["default"]
    script: |
      export thehost=$(curl -s http://example.com/next-host)
    options: {local: true}
    
  - name: run on host
    targets: ["$thehost"]
    script: |
      echo "doing something on $thehost"

This example indicates the essence of the change. Here the exported variables that are usually available for all the commands in the task will be available for all task targets and targets: ["$thehost"] will be evaluated

A practical use case for dynamic targets arises during the provisioning of a new host, followed by the execution of commands on it. Since the IP address of the new host isn't known beforehand, dynamic retrieval becomes essential.