Add mongo requirement.
How to expose ports for an application.
Need to mention all nodes to ensure flocker-deploy communicates with all
of them.
Ports are accessible regardless of node.
Data doesn't move with the application by default.
Address review comments: Phrasing improvements.
Address review comments Add requirement for mentioning all nodes to where it is first applicable.
Address review comment: Better explanation of why we're talking about data not being moved.
Address review comment: Remove demonstration that data doesn't move without volumes.
Merge remote-tracking branch 'refs/remotes/origin/master' into tutorial-ports-310
Merge pull request #344 from ClusterHQ/tutorial-ports-310

Tutorial ports.

Fixes #310.