refs #812 Next-server can be optional in the subnet, deletes the line of next-server in getAllSubnets

tests/API-dhcp/robot/API.robot

@@ -50,7 +50,6 @@ Get All Subnets
     FOR    ${subred}    IN    @{json['message']}
         Dictionary Should Contain Key    ${subred}    id
         Dictionary Should Contain Key    ${subred}    subnet
-        Dictionary Should Contain Key    ${subred}    next-server
         Dictionary Should Contain Key    ${subred}    boot-file-name
     END
 
@@ -114,7 +113,7 @@ Modify an existing subnet by id
     ${modified_subnet}=      Create Dictionary    
     ...     mask=255.255.192.0    
     ...     address=192.168.1.0    
-    ...     next-server=192.168.1.1   
+    ...     nextServer=192.168.1.1   
     ...     bootFileName=pxelinux.0
     # Modificar la subred con id=2
     ${response}=        PUT     ${BASE_URL}/subnets/2      json=${modified_subnet}     headers=${headers}      expected_status=200