Add a 'loglevel' variable to values files

Example of changing the log level when loading a chart:

    helm install --set loglevel=INFO --name voltha voltha

Change-Id: I277610758b79ea3bcf083899d9dfcf28b32e9e0c
diff --git a/voltha/templates/_helpers.tpl b/voltha/templates/_helpers.tpl
index 44b0103..46046c3 100644
--- a/voltha/templates/_helpers.tpl
+++ b/voltha/templates/_helpers.tpl
@@ -55,6 +55,6 @@
 loggers:
   '':
     handlers: [console, localRotatingFile, kafka]
-    level: DEBUG
+    level: {{ .loglevel }}
     propagate: False
 {{- end }}