add preflight check for hostnames

[SimonTheLeg]

What this PR does / why we need it:

Since some OS allow hostnames that are not valid kubernetes nodes names, this preflight check signals to the user, if some of their hostnames cannot be used as kubernetes nodes.

Which issue(s) this PR fixes:

Relates to #3071

What type of PR is this?

/kind feature

Special notes for your reviewer:

• I am not sure if there is a more elegant way in Go to combine all the hostnames into the string slice. Maybe there is a more clever way to do this?
• I have used allErrs as a string slice instead of using []error, so we don't have to import the stdlib package when then combining the errors. I think this is the more convenient way, because otherwise we would have to rename a bunch of existing imports

Does this PR introduce a user-facing change? Then add your Release Note here:

Hostnames are checked against Kubernetes node name requirements

Documentation:

NONE

[kron4eg]

/approve
/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 3116909a23f705f703c9156fa683d0cd7e071336

[kubermatic-bot]

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