BUGFIX - --start-at-task= works only with --step

From issue #2820, --start-at-task does not actually run tasks
unless --step is specified. This appears to be because skip_task
is being evaluated as True in PlayBook._run_task(). This patch
ensures skip_task is set to False in the callback.
Merge pull request #2841 from jimi1283/issue_2820

BUGFIX - --start-at-task= works only with --step