Add run_once task keyword to cause a task to bypass the host loop

This pull request adds a `bypass` task key, to allow a task to funciton like a bypass host loop plugin similar to add_host/pause/etc...

This was inspired by:
https://groups.google.com/forum/#!topic/ansible-project/3-LaugNHe0M

Example:

###### bypass.yml
```
---
- hosts: all
  tasks:
    - debug:
        var: inventory_hostname
      bypass: True
```

###### output

```
PLAY [all] ********************************************************************

GATHERING FACTS ***************************************************************
ok: [localhost]
ok: [srv1.example.org]
ok: [srv2.exampe.org]
ok: [srv3.example.org]

TASK: [debug ] ****************************************************************
ok: [localhost] => {
    "inventory_hostname": "localhost"
}

PLAY RECAP ********************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0
srv1.example.org           : ok=2    changed=0    unreachable=0    failed=0
srv2.example.org           : ok=2    changed=0    unreachable=0    failed=0
srv3.example.org           : ok=2    changed=0    unreachable=0    failed=0
```