Check ASG name list not empty

[JoelSpeed]

Fixes #278

I have built and tested this on one of our clusters.

If no autoscaling group matches both tags, the autoscaler exits and is restarted by kubernetes.

As soon as one ASG matches both tags, CA will stay up and run as designed. If this group is then removed (by changing the tags as described in the issue), then CA will also exit and get into a restart loop as above.

I considered returning an empty list of ASGs, but I believe this isn't the correct approach. If you return an empty list of ASGs then CA will continue through it's working process and only when it gets to the point where it wants to scale up or scale down will it fail. I feel users would prefer to know CA can't do anything earlier than this.

I also considered the fact that maybe CA should just emit a message and keep running and keep trying to build the cloudprovider on an interval so that it doesn't constantly restart the container (given it's run as a deployment, it will be in constant crash loops), but this requires discussion and is beyond the scope of this PR.

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[JoelSpeed]

Had to sign CLA

[mwielgus]

cc: @mumoshu

[mumoshu]

@JoelSpeed Thank you very much for your efforts!

I agree to the approch you've chosen from the three alternatives. The change LGTM.

I also considered the fact that maybe CA should just emit a message and keep running and keep trying to build the cloudprovider on an interval so that it doesn't constantly restart the container

I agree with you here, too. Although I also like it as a longer term improvement, it should be an another story as you've noted.

[mumoshu]

@mwielgus I'd appreciate it if you could release a new version of CA with this fix, probably by bumping the patch number, v0.6.1?

[mwielgus]

@mumoshu Yes, we can release it (as 0.6.2). Thanks for reviewing.

[mwielgus]

/lgtm

[mumoshu]

@mwielgus Any chance you could release this as 0.6.2? 😉
As of today, perhaps we have to upgrade K8S to 1.8.x to use the only release exists with this fix included(=CA 1.0)?