Fix buildbot upload of vs_addin archive.

This change adddresses a couple of issues with the upload
of the vs_addin archive to google storage.

Firstly is disables the upload step for periodic builders
so that they don't clobber existing uploads.

Secondly we check for the existence of a file on google
storage before uploading and error out if one is already

Review URL:

git-svn-id: 050acbb0-2703-11df-ab0a-9f3f633ae91d
1 file changed