webui: Improve bulk edit view after Hyper-V merge

Do not make platform and license key editable for allocated nodes.
Simplify code around the target platform

Also use pretty target platform in more places.
webui: Only show license key if target platform has one
Add CrowbarService.require_license_key? helper

This avoids having to reimplement the same check. It might be an easy
check for now, but it could become more complex later on.
cleanup: Use blank?
webui: Do not allow changing target platform on allocated nodes
Merge pull request #677 from vuntz/fix-bulk-edit

Several fixes related to how we present target platform in webui