Validate the hostnames at creation time

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>