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
Fix several misspells

Change-Id: I5bdac6ace595eb1ad67e8c3ab8ee34cd986ff167
Don't overwrite deployments metadata in Server resource

Currently, when metadata_update is called and local metadata has been
added for software deployments (e.g stuff not defined in the template)
we will silently discard the non-template-defined stuff when calling
metadata_update.

Arguably we are abusing the metadata section by pushing arbitrary additional
data into it but since that issue has already happened, as part of the
SoftwareDeployments implementation, the simplest fix seems to be to merge
the current and template re-resolved state of the metadata.

Change-Id: Id68d11b584734fbb828aabaca6cc75aab0f4ee4c
Closes-Bug: #1337772
Fixing a typo

Just a simple fix for a typo in a comment.

Change-Id: Ie5c165b3775601680b28d0e85fee8e4082735ffe
Merge "Implement swift client plugin"
Merge "Fix several misspells"
Merge "Fixing a typo"