refine service broker test

1. split service broker related utility function to service_broker_helper.rb
2. create service_auth_token on the fly using admin account, pending if admin
password environment variable VCAP_BVT_ADMIN_USER_PASSWD is not set.
3. enhance service filtering using vendor name.

Change-Id: Ifaee2ceb32238ceb4e119a550fa7d5ca24ae3d9d
Reviewed-on: http://reviews.cloudfoundry.org/13530
Tested-by: CI Master <cf-ci@rbcon.com>
Reviewed-by: Andrew Liu <aliu@vmware.com>
Reviewed-by: Nicholas Kushmerick <nicholask@vmware.com>