cache the pip tarball

there is no need to download the pip tarball over and over again
on rerun. Use the cached value.

Change-Id: Ibcc1807ba2aca062f69839212f05ac9394db9448
diff --git a/tools/install_pip.sh b/tools/install_pip.sh
index 9fa161e..1eb9e7a 100755
--- a/tools/install_pip.sh
+++ b/tools/install_pip.sh
@@ -71,12 +71,13 @@
 }
 
 function install_pip_tarball {
-    (cd $FILES; \
-        curl -O $PIP_TAR_URL; \
-        tar xvfz pip-$INSTALL_PIP_VERSION.tar.gz 1>/dev/null; \
-        cd pip-$INSTALL_PIP_VERSION; \
-        sudo -E python setup.py install 1>/dev/null; \
-    )
+    if [[ ! -r $FILES/pip-$INSTALL_PIP_VERSION.tar.gz ]]; then
+        (cd $FILES; \
+            curl -O $PIP_TAR_URL; \
+            tar xvfz pip-$INSTALL_PIP_VERSION.tar.gz 1>/dev/null)
+    fi
+    (cd $FILES/pip-$INSTALL_PIP_VERSION; \
+        sudo -E python setup.py install 1>/dev/null)
 }
 
 # Show starting versions