Support GLOG Severity levels

[atopuzov]

• Based on https://github.com/golang/glog/blob/23def4e6c14b4da8ac2ed8007337bc5eb5007998/glog.go#L108

I get errors of unknown severity levels when using fleuntd with syslog output (similar to [1] but forwarding to syslog instead to elastic search)

2017-11-28 12:37:53 +0000 [warn]: emit transaction failed: error_class=ArgumentError error="'I' is not a designated severity" tag="kube-proxy"
  2017-11-28 12:37:53 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/syslog_protocol-0.9.2/lib/syslog_protocol/packet.rb:72:in `severity='
  2017-11-28 12:37:53 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/remote_syslog_logger-1.0.3/lib/remote_syslog_logger/udp_sender.rb:19:in `initialize'
  2017-11-28 12:37:53 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluent-plugin-remote_syslog-0.3.3/lib/fluent/plugin/out_remote_syslog.rb:44:in `new'
  2017-11-28 12:37:53 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluent-plugin-remote_syslog-0.3.3/lib/fluent/plugin/out_remote_syslog.rb:44:in `block in emit'

[1] https://github.com/kubernetes/kubernetes/blob/master/cluster/addons/fluentd-elasticsearch/fluentd-es-configmap.yaml