Consistent sort order for minikube addons list

[zcahana]

Is this a BUG REPORT or FEATURE REQUEST? (choose one):
FEATURE REQUEST

What happened:
When invoking minikube addons list, the order in which addons are displayed is inconsistent, and varies from one invocation to another.

For example:

zvic@zvic-vbox:~$ minikube addons list
- dashboard: disabled
- default-storageclass: enabled
- coredns: disabled
- heapster: disabled
- registry: disabled
- addon-manager: enabled
- ingress: disabled
- registry-creds: disabled
- kube-dns: enabled
zvic@zvic-vbox:~$ 
zvic@zvic-vbox:~$ minikube addons list
- default-storageclass: enabled
- coredns: disabled
- kube-dns: enabled
- ingress: disabled
- registry-creds: disabled
- addon-manager: enabled
- dashboard: disabled
- heapster: disabled
- registry: disabled

What you expected to happen:
It would be nicer if the addons are printed in a consistent sort order.
The simplest approach would be to sort them according to addon name, lexicographically.

If you're ok with this change, I'd be happy to submit a PR.

[zcahana]

The proposed change is small enough so I went ahead and submitted a PR for it (#2446).