The initial airfields will be built at the rail heads. However if Vichy French land is not to be used for supply (which Petain agreed to in OTL with Tunis) it will have to be a bit further West in Turkey. Forward air bases can be built nearer the caucasus and supplied by air if need be or on the Northern Turkish coast and supplied by sea. In movement into the Caucasus may spark risings by the Azirs and Chechens etc. In 1942 in the German troops manged to reach the peak of the Caucasus coming from the North. So it is all very possible in this case with extra Apline troops and airpower.