gentoo/guest: #8406 support systemd when changing hostname

plugins/guests/gentoo/cap/change_host_name.rb

@@ -9,8 +9,14 @@ module VagrantPlugins
             basename = name.split(".", 2)[0]
             comm.sudo <<-EOH.gsub(/^ {14}/, "")
               # Set the hostname
-              hostname '#{basename}'
-              echo "hostname=#{basename}" > /etc/conf.d/hostname
+
+              # Use hostnamectl on systemd
+              if [[ `systemctl` =~ -\.mount ]]; then
+                systemctl set-hostname '#{name}'
+              else
+                hostname '#{basename}'
+                echo "hostname=#{basename}" > /etc/conf.d/hostname
+              fi
 
               # Remove comments and blank lines from /etc/hosts
               sed -i'' -e 's/#.*$//' /etc/hosts