wsk activation list behavior

A recent change to the way activations are listed produced two different behaviors.

When there are no activations, the result is this:
```
> wsk activation list
activations
```

and when repeated with activations, the result is this:
```
> wsk activation list
Datetime Activation ID Kind Start Duration Status Entity
2019-03-14 15:25:40 1d11658e3f974e6391658e3f97be63ac nodejs:6 cold 69ms success guest/e:0.0.1
```

@larandersson perhaps we should nuke the naked activations headers when there are no results.

additionally, can we trim the lines so that they don't overflow unnecessarily?