Some projects have multiple policy files for different parts of their
API. Make the sample generator extension support this by letting the
policy_generator_config_file option be set to a list of tuples mapping
the config file to the output file base name, as we do in the sample
generator in oslo.config.
Change-Id: I0c7fa409a1ed0f49d65c9b90b71317066f6d3505
Signed-off-by: Doug Hellmann <doug@doughellmann.com>