JOHANNESBURG — Oscar Pistorius will be released from prison for good behavior on Aug. 21 and go under house arrest after serving just 10 months for killing girlfriend Reeva Steenkamp, South Africa’s department of corrections said today.“He’s behaving himself very well,” Acting National Commissioner of Correctional Services Zach Modise told The Associated Press. “He hasn’t given us any problems.”Yet three months after he leaves Kgosi Mampuru II prison in Pretoria, likely for …