MB-10268: Use environment variable for user/password

Instead of providing sensitive values from commandline arguments,
environment variables CB_REST_USERNAME and/or CB_REST_PASSWORD
can be used.

Change-Id: Ib6621e23b0910b8b12bbdf4dc93643c7f0020fb4
Reviewed-on: http://review.couchbase.org/36101
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>