releasetools: Set default stdout and stderr in common.Run().

stdout and stderr will default to subprocess.PIPE and subprocess.STDOUT
respectively (which is the expected behavior from most of the existing
callers), unless caller specifies any of them.

Test: `m dist`
Test: python -m unittest \
          test_common \
          test_add_img_to_target_files \
          test_ota_from_target_files \
          test_validate_target_files
Change-Id: I43b3f08edfa8a9bcfe54baf9848dc705c048e327