packaging: setup: fixing hostname validation

engine-setup validates hostnames to avoid strange behaviours;
it's done via a regular expression.
Fixing it to avoid the match of '_' which is
not a valid character in a hostname according to RFC 952
An invalid hostname can cause strange behaviour interacting
with other components.

Change-Id: Id5c56bc5642bfb18629c1a0ad780e8a0e58a3531
Bug-url: https://bugzilla.redhat.com/1113974
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>
(cherry picked from commit 98079e8c37e00349ce5b1f86d2501a15615a3339)