Implement swift client plugin

This moves the client creation code out of Clients._swift() into
its own client plugin.

Importing swiftclient exceptions from the plugin is an interim
solution until the client plugin mechanism provides a way of
handling client-specific extensions.

Swift is a core project, and python-swiftclient is a dependency,
so the import is now mandatory.

Change-Id: Ib2cff83096639855eaa1e05febbb4b490edd6250
Merge "Implement swift client plugin"