Allow referring to activations by shorter hash ids

I would like to be able to refer to an activation id in the CLI without having to specify the full hash. It would be great to support this the way Docker does, allowing a short name if possible - like this.

```
ibmcloud@ubuntu:~$ wsk activation list hello
activations
87cd651b52d845fe912071b5c52d8dc2 hello
73fd4ac190724b008db6174c359892ab hello
5d19beb45ada418a88a96f04ffe1f77d hello
63e845cb8f2e497494c7f127c5b9317f hello
ibmcloud@ubuntu:~$ wsk activation get 87cd
error: invalid activation id (code 1167307)

```