docs/backends/cloud.rst: cosmetics and caveat about support for 'create-container'. refs #1971

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Azure: change 'container_name' config entry to 'container' for consistency with OpenStack.
Also fix a hidden bug in a test.

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>