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error out empty hostname
**What this PR does / why we need it**:
For linux, the hostname is read from file `/proc/sys/kernel/hostname` directly, which can be overwritten with whitespaces.
Should error out such invalid hostnames.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixeskubernetes/kubeadm#835
**Special notes for your reviewer**:
/cc luxas timothysc
**Release note**:
```release-note
nodes: improve handling of erroneous host names
```
cb1ef9f7e8