'before' policies don't work for join tasks

[Oleg Ovcharuk]

Let's consider this example:
version: "2.0"
wf1:
  tasks:
    task1:
      action: std.echo output=1
      on-success: task2
    task2:
      pause-before: True
      action: std.noop
      join: all

Expected result: workflow will pause before task2
Actual result: task2 ignores pause-before policy

This behavior is actual for all xxx-before policies.

Without 'join:all' everything works nice.