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/values.yaml b/xos-core/values.yaml
index 0bcc350..044317c 100644
--- a/xos-core/values.yaml
+++ b/xos-core/values.yaml
@@ -42,6 +42,8 @@
 global:
   registry: ""
 
+loglevel: "DEBUG"
+
 # NodePorts
 xos_chameleonNodePort: 30006
 xos_toscaNodePort: 30007