Design doc for SWIFT support

[arttor]

Write proposal for new data model to support both swift and s3 storages in chorus. Try to cover following topic:

• what changes to data model are needed to suppurt SWIFT's project/account/container structure?
• what will be migration path? the best option is to not have migration. just add new fields for swift and adjust code to work with both but i don't know if it will be possible.
• how hard it would be to support migration between SWIFT container and S3 bucket? Or should it be swift<->swift and s3<->s3 only? Maybe we can support it and note that not all data can be mapped (tag,acl,lifecycle,etc...). But this topic can be a part of separate document