mute useless console output, when run ./stack.sh

Run ./stack.sh will dump ~400 lines of information, because of
tar xvfz pip-*.tar.gz, and python setup.py install.

We'd better mute stdout for the two steps, to make console cleaner

Change-Id: Icf87947e020acb48d8cbe4cdcc1641f060e50f6d
diff --git a/tools/install_pip.sh b/tools/install_pip.sh
index 940bd8c..455323e 100755
--- a/tools/install_pip.sh
+++ b/tools/install_pip.sh
@@ -72,9 +72,9 @@
 function install_pip_tarball() {
     (cd $FILES; \
         curl -O $PIP_TAR_URL; \
-        tar xvfz pip-$INSTALL_PIP_VERSION.tar.gz; \
+        tar xvfz pip-$INSTALL_PIP_VERSION.tar.gz 1>/dev/null; \
         cd pip-$INSTALL_PIP_VERSION; \
-        sudo python setup.py install; \
+        sudo python setup.py install 1>/dev/null; \
     )
 }