Historically Neutron had API extensions to make it clear what feature
is available and what is not. There are however features without new
API extensions, see [1] and [2].
To avoid having lots of config options that makes it possible in a
branchless tempest to detect if a given feature is available or not on
a given Neutron branch this patch proposes a list config option similar
to api_extensions: available_features.
available_features is a list option with the special all entry, and new
tests that depend on API extensionless features can use this to be
skipped on older branches.
[1] https://review.opendev.org/750355
[2] https://review.opendev.org/743695
Change-Id: I9d4be68db0f18dcaded4bfe78f5a314cb450987a
f9bb8b8b21