Add --concurrency option to specify number of multiple actions to perform at a time.

I added --concurrency (-c in shorthand) option.
This patch does not break current behaviour.

## Examples

```
$ kitchen <action> -p
```
All actions will be performed in parallel. (same as current behaviour)

---

```
$ kitchen <action> -c 2
```
Only two actions will be performed in parallel.

---

```
$ kitchen <action>
```
Only one action will be performed at a time.