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This patch adds validation of new names used, i.e. at cluster init time,
node add time, and instance creation.

For instances, especially when using «--no-name-check» (which skips DNS
checks), we should validate the give name, and also normalize it
(otherwise, we could have two instances named inst1 and Inst1).
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>