[ONOS-7162] Forcing provider scheme to always be in lowercase.

Change-Id: I27fa58b044da7a4f6d447082ecb92c8f448152fe
diff --git a/core/api/src/main/java/org/onosproject/net/provider/ProviderId.java b/core/api/src/main/java/org/onosproject/net/provider/ProviderId.java
index ca56232..15972c9 100644
--- a/core/api/src/main/java/org/onosproject/net/provider/ProviderId.java
+++ b/core/api/src/main/java/org/onosproject/net/provider/ProviderId.java
@@ -75,7 +75,7 @@
      * @param ancillary ancillary provider indicator
      */
     public ProviderId(String scheme, String id, boolean ancillary) {
-        this.scheme = checkNotNull(scheme, "Scheme cannot be null");
+        this.scheme = checkNotNull(scheme, "Scheme cannot be null").toLowerCase();
         this.id = checkNotNull(id, "ID cannot be null");
         this.ancillary = ancillary;
     }