fix #4460: setup: centos: create /etc/hostname if it does not exist

fix #4460: setup: centos: create /etc/hostname if it does not exist

Previously, the CentOS setup only wrote to /etc/hostname if the file
already existed. Many CT templates of Redhat-derived distros do not
contain that file, so the containers ended up without /etc/hostname.
This caused systemd-hostnamed to report the "static hostname" to be
empty. If networking is handled by NetworkManager, the empty static
hostname caused DHCP requests to be sent without the "Hostname"
field, as reported in #4460.

With this fix, the CentOS setup module creates /etc/hostname if it
does not exist, so NetworkManager correctly reads the hostname and
includes it in DHCP requests.

Manually tested with the following CT templates (checking that
/etc/hostname exists and DHCP requests include the hostname):

- Distros using NetworkManager:

  - Alma Linux 9 (almalinux-9-default_20221108_amd64.tar.xz)
  - CentOS 8 (centos-8-default_20201210_amd64.tar.xz)
  - CentOS 9 Stream (centos-9-stream-default_20221109_amd64.tar.xz)
  - Rocky Linux 9 (rockylinux-9-default_20221109_amd64.tar.xz)

- Distros using network-scripts (here, DHCP requests already
  contained the hostname without this fix, as network-scripts does
  not rely on systemd-hostnamed):

  - Alma Linux 8 (almalinux-8-default_20210928_amd64.tar.xz)
  - CentOS 7 (centos-7-default_20190926_amd64.tar.xz)
  - CentOS 8 Stream (centos-8-stream-default_20220327_amd64.tar.xz)
  - Rocky Linux 8 (rockylinux-8-default_20210929_amd64.tar.xz)

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>
 [ T: slightly touch up of commit message format / wording ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>