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 manilaclient code to support the last case (ie:
os_cert and os_key are non-empty) with the --os-key option
and OS_KEY environment variable.
Closes-Bug: #1999775
Change-Id: I09313b7c55a5f9d4ec032a37e69a1d79db29d648