An operator proposing a web UX to its customers might want to allow web
browser to access some headers by default (eg: X-Storage-Policy,
X-Container-Read, ...). This commit adds a new setting to the
proxy-server to allow some headers to be added cluster-wide to the CORS
header Access-Control-Expose-Headers.
Change-Id: I5ca90a052f27c98a514a96ee2299bfa1b6d46334