use external_openstack_lbaas_use_octavia for template openstack-cloud… (#7298)

* use external_openstack_lbaas_use_octavia for template openstack-cloud-config

* Delete external_openstack_lbaas_use_octavia from default values. Added description and default values of variables to docs

* markdown fix

* make this simple

* set external_openstack_lbaas_use_octavia in default values

* duplicated variable in doc