build process outputs each line twice

[Vladislav Belogrudov]

when running kolla-build or tools/build.py like this:
   tools/build.py --config-file etc/kolla/kolla-build.conf
the build process outputs each line twice, e.g.:

INFO:kolla.image.build.base:Step 14 : COPY influxdb.yum.repo /etc/yum.repos.d/influxdb.repo
INFO:kolla.image.build.base:Step 14 : COPY influxdb.yum.repo /etc/yum.repos.d/influxdb.repo
INFO:kolla.image.build.base: ---> Using cache
INFO:kolla.image.build.base: ---> Using cache
INFO:kolla.image.build.base: ---> 61589053a46c
INFO:kolla.image.build.base: ---> 61589053a46c

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/365567

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/365567
Committed: https://git.openstack.org/cgit/openstack/kolla/commit/?id=80bdb458eef50aa2a5499555a73b9b70cc6fb63a
Submitter: Jenkins
Branch: master

commit 80bdb458eef50aa2a5499555a73b9b70cc6fb63a
Author: Vladislav Belogrudov <email address hidden>
Date: Mon Sep 5 13:29:50 2016 +0300

    Build process outputs each line twice

    Don't propagate logs going to stdout to avoid duplicate messages

    Co-Authored-By: Jeffrey Zhang <[e-mail]>
    Change-Id: I52da42adc40aa5d468d1c0c1134e65689916a2a8
    Closes-Bug: #1620239