If OS_VOLUME_API_VERSION is not set, use the highest
supported by both the client and the server.
If OS_VOLUME_API_VERSION exceeds that supported by the server,
use the highest supported by both the client and the server.
A warning message is printed for the user indicating that this
happened.
(This is similar to the behavior of the manila CLI, and is
mostly code from manilaclient tweaked to work in cinderclient.)
Change-Id: Ie1403eca2a191f62169e60c0cde1622575327387