Add a discoverable_apis option for swift

This commit adds a new list option for the object-storage-feature-enabled
config group, discoverable_apis, which lists out which Swift discoverable
optional apis are expected to be enabled on the swift server. It also
converts the pre-existing single boolean options for these types of
features to use the new list option. This option is used in the same
manner as the extensions list options for the other services. However,
since swift doesn't have the same concept of extensions as some of the
other OpenStack projects optional discoverable apis (which are often
but not always tied with middleware) were used instead. If an optional
api being enabled isn't discoverable from an external api call then
that should have a separate config option in the
object-storage-feature-enabled config group.

Partially implements bp config-cleanup

Change-Id: I846860804050ec44491ce05d6b72e1cc639609b1
Merge "Add a discoverable_apis option for swift"