[install] Add more content about SERVICE_DBPASS
[install] Add more content about SERVICE_DBPASS

backport: ocata newton mitaka

Change-Id: I1d6eae9c3aff8e8af9996f6414ca21dce4cb7171
Closes-Bug: #1689242

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals