add support of task's targets set directly in playbook

[umputun]

Add new task-level filed targets.

It will be used only in case no -t/--target is set in the command line. Technically, -t always set as it has a default target default; however, this PR considers such default as non-set and will use task.targets

This is what it looks like in the playbook:

targets:
  prod:
    hosts: [{host: "h1.example.com", user: "test"}, {"h2.example.com", "port": 2222, name: "h2"}]
  staging:
    groups: ["staging"]

tasks:
  - name: task1
    targets: ["dev", "host3.example.com:2222"]
    commands:
      - name: command1
        script: echo "Hello World."

targets inside the task resolved the same way as targets defined in the command line, i.e. all the sequence of matching groups, tags, names, etc...

The reason why this may be handy is the ability to make a playbook with multiple tasks where all the tasks are sort of related but should be running on a different subset of hosts. For instance, one task deploys changes to the master/controller boxes, and another task deploys to working nodes.

Before this change, It was possible to run the spot twice for different task with different targets or even different playbooks. However, for related tasks, it is very nice to have a single spot run dealing with everything