allow relative paths in run_external_tasks, resolving against parent base path/URL

[pk910]

this PR allows adding external playbooks with relative paths like this:

- name: run_external_tasks
  title: "Test relative include"
  timeout: 30m
  config:
    testFile: ../some_path/test.yaml

relative paths are resolved relative to the parent base directory, which allows nested playbooks with relative paths even from URLs.

eg. if the above playbook is loaded from http://example.com/my_tests/test.yaml, the referenced external_tasks playbook is loaded from http://example.com/some_path/test.yaml (http://example.com/my_tests/../some_path/test.yaml)