Update kube-proxy to handle aws returning multiple hostnames

[erickt]

Amazon's dhcp service supports returning a space separated list of hostnames. This patch makes sure to split the domains first before picking the hostname override.

Closes #3060.

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[chrislovecnm]

@justinsb does it matter which host name we use?

[chrislovecnm]

/ok-to-test

[erickt]

The other way I suppose is skipping the aws metadata server and Looking up the fully qualified domain name (which kops might already be doing?). I'm not sure how to do a getaddrinfo in go but I did find this stack overflow about how to do it.

[justinsb]

So the node name does have to match the privatednsname, but we also override the node name in the aws cloud provider e.g. https://github.com/kubernetes/kubernetes/blob/master/pkg/cloudprovider/providers/aws/aws.go#L1496-L1503

I suggest getting this merged as it fixes a clear issue, and then seeing if picking the first name goes wrong. I don't think it will as I think this name is now mostly not used as a key but rather simply for information / diagnostics. I think.

Out of interest @erickt , what is the use case for multiple hostnames in DHCP? Seems likely to cause problems :-)

[rmazlumyan]

@justinsb. In my case, defining multiple domain-name entries in vpc dhcp option set seems to result in not only the /etc/resolv.conf search field getting populated by all the domains (desired behaviour) but also concatenation of entire domain including spaces to hostname in meta-data local-hostname (undesirable).

[justinsb]

/lgtm

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