Created by: wojtuss

With this patch, MKL-DNN is enabled for test_slim_int8_* tests via FLAGS_use_mkldnn=true env variable, i.e. the FP32 part of the test runs with MKL-DNN when FLAGS_use_mkldnn=true, the INT8 part runs with MKL-DNN either way.

Also, a test case with MKL-DNN is added for MobileNetV1.

test=develop