corrected output: "pip3 install pymongo" for Python 3, "pip install

pymongo" for Python 2

Change-Id: I34e630b8f76e51e794232ad5f121d2bfbfad0d32

agents/plugins/mk_mongodb.py

@@ -35,7 +35,7 @@
 except ImportError:
     sys.stdout.write("<<<mongodb_instance:sep(9)>>>\n")
     sys.stdout.write(
-        "error\tpymongo library is not installed. Please install it on the monitored system (pip install pymongo)\n"
+        "error\tpymongo library is not installed. Please install it on the monitored system (for Python 3 use: 'pip3 install pymongo', for Python 2 use 'pip install pymongo')\n"
     )
     sys.exit(1)