install_arch: init pacman keyring on first boot

Creates a systemd service which executes these commands on first boot:

pacman-key --init

pacman-key --populate archlinux

Also it makes sure /etc/pacman.d/gnupg is not absent.

At last it removes /etc/machine-id to make sure systemd launches first
boot services.

roles/install_arch/files/pacman-init.service

+[Unit]
+Description=Initializes Pacman keyring
+Before=sshd.service
+ConditionFirstBoot=yes
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/bin/pacman-key --init
+ExecStart=/usr/bin/pacman-key --populate archlinux
+
+[Install]
+WantedBy=multi-user.target

roles/install_arch/tasks/main.yml

@@ -150,8 +150,17 @@
   register: chroot_grub_mkconfig
   changed_when: "chroot_grub_mkconfig.rc == 0"
 
+- name: setup pacman-init.service on first boot
+  copy: src=pacman-init.service dest=/mnt/etc/systemd/system/ owner=root group=root mode=0644
+
+- name: remove generated keyring in the installation process
+  file: path=/mnt/etc/pacman.d/gnupg state=absent
+
+- name: make sure /etc/machine-id is absent
+  file: path=/mnt/etc/machine-id state=absent
+
 - name: enable services inside chroot
-  command: chroot /mnt systemctl enable sshd systemd-networkd systemd-resolved fstrim.timer
+  command: chroot /mnt systemctl enable sshd systemd-networkd systemd-resolved fstrim.timer pacman-init
   register: chroot_systemd_services
   changed_when: "chroot_systemd_services.rc == 0"