Add DataNode list to the SCM WebUI

Currently on the SCM Web UI we have a table that provides information about the status of the DataNodes in a way that is summarizes the number of nodes in different states, and represent the results in a table.

In this JIRA I would like to propose to remove this table, and introduce a new approach to show the information, and open the possibility to add new things easily.

If we show the DataNode host name in the first column, then we have the possibility to provide information on a per DataNode basis, similarly as we have in the HDFS NameNode UI. With the possibility to filter and sort the table contents, and proper pagination, the table can provide any kind of easily accessible DN information that SCM is aware of.

Changing the table to this format, allows us to link from the SCM UI to the DN UI, it allows us to provide various other information about the DataNodes, and also it contain more useful information about which node is in which state instead of showing just an aggregate. The links come in handy for example for integrating the DN WebUIs as well with Knox, after KNOX-2833. I know Recon can provide the same, and already has a similar table, but all in all Recon is an optional component, so we should provide at least a subset of the information shown in Recon on other UIs.

The initial aim covered in this JIRA is to show the DN hostname as a link to the DN WebUI in the first column of the new table, then show the health and decommissioning state of the DN in the 2nd and 3rd columns leaving the same information available. It is out of scope to add new things, those might come later on.