CBQE-0: fix error python_exe not found

Change-Id: I1e65d948243b14fa715097a941803024628a3ce3
Reviewed-on: http://review.couchbase.org/37679
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Reviewed-by: Meenakshi Goel <meenakshi.goel@globallogic.com>
Tested-by: Meenakshi Goel <meenakshi.goel@globallogic.com>