improve crushtool --build useability and documentation

* crushtool --build dumps the created crush tree to show what has actually be done, which is helps diagnose unintended results.
* a warning is displayed when there are multiple roots because it is not usualy the intent
* the crush rulesets generated are the same as OSDMap because it probably is the starting point for the user most of the time
* the documentation is reworked to include an example and better illustrate the usage