Retry yarn package installs

We're seeing occasional npm install failures in jobs; retry the
"yarn install" command if it fails.

Also, pass the YARN_REGISTRY env var in the tox remote job.

Change-Id: Ic80dccf0869ce805368f678eafcfdf48f8bf9bea

tools/pip.sh

@@ -47,7 +47,18 @@ then
             echo "Using yarn registry: ${YARN_REGISTRY}"
             sed -i "s#https://registry.yarnpkg.com#${YARN_REGISTRY}#" yarn.lock
         fi
-        yarn install --verbose
+
+        # Be forgiving of package retrieval errors
+        attempts=0
+        until yarn install --verbose; do
+            ((attempts++))
+            if [[ $attempts > 2 ]]
+            then
+                echo "Failed installing npm packages"
+                exit 1
+            fi
+        done
+
         yarn build
         if [[ -n "${YARN_REGISTRY}" ]]
         then

tox.ini

@@ -107,6 +107,7 @@ passenv =
   OS_LOG_DEFAULTS
   OS_STDERR_CAPTURE
   OS_STDOUT_CAPTURE
+  YARN_REGISTRY
   ZUUL_REMOTE_IPV4
   ZUUL_SSH_KEY
   ZUUL_TEST_ROOT