Run-time target evaluation

[umputun]

The idea is to assign the target inside the task in the runtime. The issue I'm trying to address is this: the first task makes a "thing", let's say, a new instance. To address this instance as a target for the next task, this info should be passed in. Currently, we can propagate this IP like export INSTANCE_IP=$(whatever command), but it won't make any target and will force us to use the script with the local option and make all the operations a part of the script. In addition, it won't be populated with other tasks.

The idea is to allow task's target to be a variable, i.e.

- name: copy files to the instance
   targets: [$new_instance]
   ...

To me, it sounds logical and even expected behavior. Allowing task-level targets to be vars should be easy to implement; we could do it similarly as all the vars are populated via env.

[umputun]

This was implemented by #92