docs: Add security section.
docs: Update OpenStack driver example, make sure base url also contains tenant
id.
Modify Connection.request to create a shallow copy of "params" and "headers" argument before mutating it.
Disable cache busting stuff in the OpenStack driver and only enable it with
Rackspace first-gen driver.

Also move base cache busting functionality on a base Connection class so it can
be more easily re-used with other drivers.
Update CHANGES.
docs: Update upgrade notes, document cache busting changes.