add support for storageengine

the information will be report as testbed.servers[0].mongod.storageengine.name, such as:
      "testbed": {
          "type": "standalone",
          "servers": {
              "mongod": [
                  {
                      "hostinfo": {
                          ...
                      },
                      "serverinfo": {
                          ...
                      },
                      "storageengine": {
                          "name": "wiredtiger"
                      }
                  }
              ]
          }
      },

if there is no such information, such as from old mongod, it will "undefined"