crush: revise JSON format for 'item' type

This PR against firefly rather than master, to avoid the intermediate version of the format ever going into the wild. 

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Commit a7e9a7b648 changed the JSON format of CRUSH rules
such that the 'item' attribute on a step was sometimes
an integer and sometimes a string.

This commit separates the integer and string representations
so that tools which rely on a 'item' consistently being an
integer ID will work.

Signed-off-by: John Spray <john.spray@inktank.com>