Nexus vrf support (#246)

Initial commit to add the suport of the Interfaces in VRF for Nexus devices
To tune to correctly retrieve the context for the VRF

On Nexus device, you need to add the following command for each VRF:
snmp-server context <context> vrf <vrf>

lib/SNMP/Info/Layer3/Nexus.pm

@@ -56,7 +56,8 @@ $VERSION = '3.39';
 
 %MIBS = (
 	%SNMP::Info::Layer3::CiscoSwitch::MIBS,
-	'CISCO-ENTITY-VENDORTYPE-OID-MIB' => 'cevMIBObjects',
+	'CISCO-ENTITY-VENDORTYPE-OID-MIB'   => 'cevMIBObjects',
+    'CISCO-CONTEXT-MAPPING-MIB'         => 'cContextMappingMIBObjects',
 );
 
 %GLOBALS = (
@@ -64,7 +65,10 @@ $VERSION = '3.39';
 	'mac' => 'dot1dBaseBridgeAddress',
 );
 
-%FUNCS = ( %SNMP::Info::Layer3::CiscoSwitch::FUNCS, );
+%FUNCS = ( 
+    %SNMP::Info::Layer3::CiscoSwitch::FUNCS, 
+    'vrf_name' => 'cContextMappingVrfName',
+);
 
 %MUNGE = ( %SNMP::Info::Layer3::CiscoSwitch::MUNGE, );