Feature/better params

A long time ago, in a galaxy far far away, we added support for Stories to pass settings into what became StoryTemplates.  These settings are known as 'params'.  Since then, we've added a -D switch to allow us to override the params from the command-line (so that we can tell the story which platform to provision onto at runtime, for example).  Things got a bit messy.

This pull request sorts out params once and for all:

* establishes a clear order of precedence
* ensures params are available in all phases of a story