Add explicit Swift enable flag
Created by: haikuginger
This change adds support for versions of configuration at or newer than https://github.com/edx/configuration/commit/5754b49810d8195975368a8167e53ac7b5cb1bb3. That commit does a meta-include of the Openstack role in edxapp when this explicit flag is set.
Dependencies: None
Testing instructions:
- Create an instance using version 5754b49 or newer of configuration.
- Ensure that the instance does not use ephemeral databases.
- Spawn an appserver, noting that it spawns successfully with Swift storage enabled.
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