Add the ability to add and remove nodes

[Matt Fuller]

Syntax: presto-admin add worker - adding a worker
Install presto on the new worker
Configure it. Use default worker configuration.
Starts Presto on this node

Syntax: presto-admin replace coordinator - adding a coordinator
Removes the old coordinator. Stops Presto on the entire cluster
Install presto on the new coordinator
The new coordinator will have to be configured. If the old coordinator is up (node is not yet terminated) then use that config, else use the default coordinator config.
Configure the workers to point to the new coordinator. Just change the coordinator ip on worker nodes (rest of the config should be unchanged)
Starts Presto on the entire cluster.

Removing a coordinator is not allowed
Syntax: presto-admin remove worker
Just stop Presto (do not uninstall it yet)
The "stop" Presto here can be a call to graceful shutdown followed by prestoadmin stop
Graceful shutdown on a node which is a worker would mean that that worker is "preparing" for shutdown and is not going to take any more queries.
This command will fail if run on a coordinator node (graceful shutdown REST call on a coordinator node fails in presto anyway)
Syntax: presto-admin remove worker --force
Stop Presto and uninstall.
Again, this will work only on worker nodes.

There are various config properties in Presto we need to tweak once the cluster size in terms of #workers. We can leave it to the user to do a call to ./presto-admin configure. This should be documented.