Currently we can specify a client certificate key using --os-key option
inherited from keystoneclient cli options but it has no effect because
keystoneclient Session expects as cert argument, one of the followings:
* None (no client certificate),
* a path to client certificate,
* a tuple with client certificate/key paths.
The change updates cinderclient code to support the last case (ie:
os_cert and os_key are non-empty) in order to take into --os-key option
and OS_KEY environment variable.
Closes-Bug: #1564646
Change-Id: I258fd554ad2d6a5413ffe778acefa3a0b83e591f