Fix partial download problem

(1) when checksum fails, better delete the broken files and try the second time;
(2) amazon s3 is not good in mainland China, better try one more time with wget

Change-Id: I24ee73f216b78bd80564863cd335e5d5a9b56360
diff --git a/lib/etcd3 b/lib/etcd3
index 60e827a..934bbb0 100644
--- a/lib/etcd3
+++ b/lib/etcd3
@@ -98,8 +98,8 @@
     etcd_file="$(get_extra_file $ETCD_DOWNLOAD_LOCATION)"
     if [ ! -f "$FILES/etcd-$ETCD_VERSION-linux-$ETCD_ARCH/etcd" ]; then
         echo "${ETCD_SHA256} $etcd_file" > $FILES/etcd.sha256sum
-        # NOTE(sdague): this should go fatal if this fails
-        sha256sum -c $FILES/etcd.sha256sum
+        # NOTE(yuanke wei): rm the damaged file when checksum fails
+        sha256sum -c $FILES/etcd.sha256sum || (sudo rm -f $etcd_file; exit 1)
 
         tar xzvf $etcd_file -C $FILES
         sudo cp $FILES/$ETCD_NAME/etcd $ETCD_BIN_DIR/etcd