Couchbase backup failing

Executing a backup on Couchbase doesn't finish. Examining the log in
the Guest Agent revealed the following error:
'OSError: [Errno 2] No such file or directory'

This occurred because a list was being passed to 'execute' instead of
*args. These instances have been fixed.

Change-Id: I4616437909a31818faf082f0c272de9415219acd
Closes-Bug: #1387721