[install] Fix the incorrect function name
[install] Fix the incorrect function name

On Ubuntu 14.04, SUSE and Debian, MongoDB shell version is 2.4.x.
We should use 'addUser' instead of 'createUser'.

backport:mitaka

Change-Id: I66178af5477b52d8e22f44d509c4af30feb4ace5
Closes-Bug:#1582356

AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals