Added a retry for the local credentials store.

An IOException can occur when multiple workers on the same machine try to create the directory to hold the stored credentials.

(Previously this directory was created in advance of running the job when credentials were manually propagated to all workers.)
Updated Google Compute Engine instructions.

Added a link to Click-to-deploy for Hadoop/Spark on GCE from @mbookman

Also simplied gcutil instructions since this job can be invoked by any user.