conf: do not generate machine names ending with a dash

conf: do not generate machine names ending with a dash

As of systemd commit:

commit d65652f1f21a4b0c59711320f34266c635393c89
Author:     Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
CommitDate: 2018-12-10 09:56:56 +0100

    Partially unify hostname_is_valid() and dns_name_is_valid()

Dashes are no longer allowed at the end of machine names.

Trim the trailing dashes from the generated name before passing
it to machined.

https://bugzilla.redhat.com/show_bug.cgi?id=1790409

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>