Update nginx sample configs to support CORS

First comment:

Originally entered By Andy as a Github issue (1226) for SG. Moved to the mobile portal issues:

Forum post was asking about configuring CORS in SG, with nginx in front of SG.
Looks like nginx may need to be configured to support CORS.

http://serverfault.com/questions/162429/how-do-i-add-access-control-allow-origin-in-nginx

From original forum post OP issue was a typo in their CORS config.

see other comments: https://github.com/couchbaselabs/couchbase-mobile-portal/issues/270