Don't default to Kubernetes, ask the user what provider to use if multiple providers are added.

At the moment we default to Kubernetes.

I propose that if we do not provide a default provider it via --provider or through the Nulecule file, we ask the user similar to parameters as well.

For example:

```
sudo atomicapp run projectatomic/helloapache

...

>> Supported providers:
>> {K}ubernetes
>> {O}penShift
>> {D}ocker
>> {M}esos

>> Please pick a current provider: O
```
