Fix "device" as the optional para on volume-attach

Attach a volume to a server without device name, the client returned the
error msg: "error: too few arguments". But for server, we can attach a
volume to a server without the device name.

Change-Id: Ia2743c6e3956cae6591ecd414baf89b0dbcf532b
Closes-Bug: #1257137
Merge "Fix "device" as the optional para on volume-attach"