Spanner: gRPC config not being passed to 'create_channel'

[larkee]

The refactor in commit googleapis/google-cloud-python@1841c7a, resulted in an unused variable.

Previously, options was used as a keyword argument in the google.api_core.grpc_helpers.create_channel call so I think it has just omitted. This is the only reference to grpc config I could find so this probably means the grpc channel is not being configured properly.