Redirect subprocess stderr to stdout in verbose mode.

This patch uses subprocess.communicate instead of subprocess.wait to
prevent deadlock if any of the child processes outputs too much data,
and redirects the subprocess output to stdout when running in verbose
mode.

With this patch `ota_from_target_files -v` prints the delta_generator
output in stdout, and no output if '-v' is not passed.

Bug: None
TEST=ota_from_target_files -v ...

Change-Id: Id66e4f3360a6f91d61a3ce96d53afbccdaa19da5