Can I use an op-amp in voltage follower configuration as variable voltage power supply?
It can be used but for most op-amps the maximum current you would be able to drive from the output will be limited to less than 50mA. Op-amps are not miracle workers! On the other hand you can attach an NPN and PNP transistor in a push-pull configuration and get more current: -


(source: northwestern.edu)  

Or, you could use an OPA541 - it is an op-amp and can supply up to 10 amps: -





I guess this one is a bit of a miracle worker!