Preserve result returned by apt-get and pip

Change I855ffe9c7a75e9943106af0f70cf715c34ae25c5 and
I368fec44858bd97fc6a314fb20eed2b10932cbb1 added timing
infrastructure which hides the return value of
the main commands.  Restore the prior behaviour.

Change-Id: I4a536abefa7824ac6d42e7bbbf1079d9cc055984
Closes-Bug: #1518545
diff --git a/functions-common b/functions-common
index 4a9db34..486dcc5 100644
--- a/functions-common
+++ b/functions-common
@@ -1045,7 +1045,7 @@
 # Uses globals ``OFFLINE``, ``*_proxy``
 # apt_get operation package [package ...]
 function apt_get {
-    local xtrace
+    local xtrace result
     xtrace=$(set +o | grep xtrace)
     set +o xtrace
 
@@ -1062,9 +1062,11 @@
         http_proxy=${http_proxy:-} https_proxy=${https_proxy:-} \
         no_proxy=${no_proxy:-} \
         apt-get --option "Dpkg::Options::=--force-confold" --assume-yes "$@"
+    result=$?
 
     # stop the clock
     time_stop "apt-get"
+    return $result
 }
 
 function _parse_package_files {