add 2018 to osreleasemajor for agent_restart_command

[cabrinha]

Amazon Linux now has a 2018 release date on their AMIs:

# cat /etc/issue

Amazon Linux AMI release 2018.03
Kernel \r on an \m

# facter -p os
{
  architecture => "x86_64",
  family => "RedHat",
  hardware => "x86_64",
  name => "Amazon",
  release => {
    full => "2018.03",
    major => "2018",
    minor => "03"
  },
  selinux => {
    enabled => false
  }
}

So, when not specifying $agent_restart_command, we get this:

Error: /Stage[main]/Puppet::Agent::Service::Systemd/Service[puppet-run.timer]: Provider systemd is not functional on this host

[ekohl]

Thanks!