python: use communicate to fix Popen deadlock

python: use communicate to fix Popen deadlock

`Popen.wait()` will deadlock when using stdout=PIPE and/or stderr=PIPE and the
child process generates large output to a pipe such that it blocks waiting for
the OS pipe buffer to accept more data. Use communicate() to avoid that.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Oct 19 09:27:16 CEST 2017 on sn-devel-144

(cherry picked from commit 5dc773a5b00834c7a53130a73a48f49048bd55e8)

Autobuild-User(v4-5-test): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(v4-5-test): Tue Nov 14 14:35:22 CET 2017 on sn-devel-144