CIMI: Fixed various inconsistencies between JSON and XML samples and revampled specs

[mfojtik]

Hi,

Thanks David for review and suggestion to move all assertions to single helper
in order to check sanity between JSON->XML and vice versa serializations.

Also I fixed various typos in XML and JSON samples and fixed wrong property
types in MachineImage and MachineConfiguration model.

I'll continue tomorrow with improving spec files (add more check but keep it DRY
as possible).

 -- Michal