Update volume status AFTER terminate_connection is done

Update volume status AFTER terminate_connection is done

Current volume API terminate_connection() updates volume's status in
DB before volume driver finishes terminate_connection().  In cases
where volume driver needs some time complete terminate_connection(),
it means volume becomes 'available' before volume driver done cleaning
up.  This fix changes the order of 'unreserve_volume' (updating DB)
and driver's terminate_connection to make sure when volume is marked
'available' the client has done cleaning connection.

Note that terminate_connection volume RPC API is a synchronized call,
but volume manager/driver doesn't return any value to caller.  What's
more, when volume API extension calls volume terminate_connection API,
it doesn't expect return values, so this change removes unnecessary
return values from volume API.

Closes-bug: #1488857

Change-Id: If79f7ce9a7d238b41be4c6befa0f3cac30ab06a3