Is it safe to put a woodstove on a sheet of cement board?
Actually there are a few considerations you need to take into effect. 

You first need to look up the "R" factor required from combustible materials for your specific stove. Every manufacturer has a safety insulation factor. This can be provided by materials such as concrete board, high density fiberboard insulation, steel sheets and air barriers.  The factors can be different from the bottom, sides and back of a stove, so it is important to have the full specs. You should also check with the local building inspector to get the minimum distances from combustibles in your area. I believe most communities use the NFPA specs, but always check local requirements.

Practically speaking, using a single piece of concrete board under a wood stove is rarely enough insulation factor. I think 1/2" concrete board only has a R-rating of 2 or 3, which is much too low if attached directly to a wood floor without an air space. I would feel much more comfortable with a layer or two of brick or brick over high temp fiberboard insulation. High temp fiberboard can be rated as high as 800 degrees F per inch at 6 inches away from heat source. Available at any HVAC supplier. 

After all the work you have done on your yurt, be absolutely safe and sure of you woodstove installation.  

Good Luck my friend.