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/xos-core/templates/_core.tpl b/xos-core/templates/_core.tpl
index e0d545b..6722475 100644
--- a/xos-core/templates/_core.tpl
+++ b/xos-core/templates/_core.tpl
@@ -42,7 +42,7 @@
         - console
         - file
         - kafka
-      level: DEBUG
+      level: {{ .Values.loglevel }}
 {{- end }}
 
 {{- define "xos-core.initial_data" }}