This adds a new tox environment, genconfig, which generates sample
neutron VPNaaS configuration file using oslo-config-generator.
Updates to some configuration option help messages to reflect useful
details that were missing in the code but were present in config files.
DocImpact: Update the docs that VPNaaS no longer includes static example
configuration files. Instead, use tools/generate_config_file_samples.sh
to generate them and the files generated now end with .sample extension.
Partially-Implements: blueprint autogen-neutron-conf-file
Change-Id: I4a6094b8218dfd320d05bfb1e3bc121e8930c551
Partial-bug: #1199963
5c8941eeed