Set useragent in Kubernetes API calls

Maxim Ivanovrequested to merge
redbaron1/gitlab-runner:kube-client-useragent into master

What does this MR do?

Sets User-Agent when making Kubernetes API requests

Why was this MR needed?

User Agent is shown in apiserver logs, makes it easier to find requests done by gitlab runners when investigating issues.

What are the relevant issue numbers?

#3459 (closed)

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