Assume systemctl as default for restarting sshd.service

This also drops support for linux systems without systemd, for example
EL6. But we don't support those anymore since a long time.

lib/beaker/host/unix/exec.rb

@@ -278,20 +278,12 @@ def clear_env_var key
   # @return [Result] result of restarting the SSH service
   def ssh_service_restart
     case self['platform']
-    when /debian|ubuntu/
-      exec(Beaker::Command.new("service ssh restart"))
-    when /amazon|(el|centos|redhat|oracle|scientific)-[7-9]|eos-7|fedora-(1[4-9]|2[0-9]|3[0-9])|archlinux-/
-      exec(Beaker::Command.new("systemctl restart sshd.service"))
-    when /el-|centos|fedora|redhat|oracle|scientific|eos/
-      exec(Beaker::Command.new("/sbin/service sshd restart"))
-    when /opensuse|sles/
-      exec(Beaker::Command.new("/usr/sbin/rcsshd restart"))
     when /solaris/
       exec(Beaker::Command.new("svcadm restart svc:/network/ssh:default"))
     when /(free|open)bsd/
       exec(Beaker::Command.new("sudo /etc/rc.d/sshd restart"))
     else
-      raise ArgumentError, "Unsupported Platform: '#{self['platform']}'"
+      exec(Beaker::Command.new("systemctl restart sshd.service"))
     end
   end