[install] make getting swift_url more clear
[install] make getting swift_url more clear

Updated the Cinder backup service install section on how to retrieve
backup_swift_url.  Previous provided value was confusing as it did not
include project_id in URL.

Change-Id: I6cc6afd467d1091bccbe97af90672c3c6ca17c4a
Closes-Bug: 1652589

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