Provide UI alert/warning related to port availability between nodes

Working with a customer recently, realized that some ports were being blocked and this was causing unpredicatble and hard-to-diagnose problems with rebalance, although the UI constantly claimed that the nodes were up and available.

It would be very helpful for the end-user if the UI presented an error/warning/alert on each node if the various necessary ports are not available between them.

As the product evolves, there are more and more ports added and not necessarily exposed clearly to the users...the UI will help notify them that there is an external configuration issue.

Ports include:
8091/8092
11210/11209
11214/11215 (the new SSL ports)
others?

This should also extend to our XDCR management screen for connections between all nodes of all clusters...it's too easy to make a mistake and very hard to determine where the problem is.