Storagedriver: GCS: add chunksize parameter

Signed-off-by: Arthur Baars <arthur@semmle.com>

docs/configuration.md

@@ -86,6 +86,7 @@ information about each option that appears later in this page.
         bucket: bucketname
         keyfile: /path/to/keyfile
         rootdirectory: /gcs/object/name/prefix
+        chunksize: 5242880
       s3:
         accesskey: awsaccesskey
         secretkey: awssecretkey

docs/storage-drivers/gcs.md

@@ -52,6 +52,17 @@ An implementation of the `storagedriver.StorageDriver` interface which uses Goog
     <td>
       This is a prefix that will be applied to all Google Cloud Storage keys to allow you to segment data in your bucket if necessary.
   </tr>
+  </tr>
+   <tr>
+    <td>
+      <code>chunksize</code>
+    </td>
+    <td>
+      no (default 5242880)
+    </td>
+    <td>
+      This is the chunk size used for uploading large blobs, must be a multiple of 256*1024.
+  </tr>
 
 </table>