ec2_asg: Initialize create_changed and replace_changed

Running ``ec2_asg`` with an already existing ``asg`` makes ansible crash if there is no value for ``replace_all_instances`` or ``replace_instances``. (i.e wating for ``viable_instances > X``)

    - name: Wait for viable_instances
      local_action:
            module: ec2_asg
            name: asg-name
            region: eu-west-1
      register: asg
      until: asg.viable_instances|int >= 2
      delay: 10
      retries: 120

**Expected behavior:** Do not crash and return the status of the ``asg``.

**Current behavior:** Crash with the following error ``UnboundLocalError: local variable 'replace_changed' referenced before assignment``.

**Fix behavior:** Expected behavior.