Add OpenStack Keystone V3 support (#4889)

Keystone V2 is deprecated in favour of V3. This adds the necessary
connection parameters for establishing a V3 connection. Connections
to V2 endpoints are still possible and the configuration should
remain compatible.

This also introduces a SWIFT_REGION variable for multi-region
OpenStack environments and a SWIFT_CACHE_TTL that controls how long
tokens and other meta-data is cached for. Caching tokens avoids
rate-limiting errors that would result in media uploads becoming
unavailable during high load or when using tasks like
media:remove_remote. fog-openstack only supports token caching for
V3 endpoints, so a recommendation for using V3 was added.

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