This dynamically adds the elasticsearch path.repo configuration
entry if it's not defined. This solves issues arising when the
storage settings are disabled in favor of emptydirs for simpler
ES deployments. If elasticsearch attempts to configure the repo
path with an invalid entry (inaccesible external or shared fs
path), the service will crash.
Change-Id: I089b77104107dfb1f8e6ea2d8a560384718e63f9
99ee859b66