[contrib/terraform/openstack] Backward compatibility changes (#2539)

* [terraform/openstack] Restores ability to use existing public nodes and masters as bastion.

* [terraform/openstack] Uses network_id as output

* [terraform/openstack] Fixes link to inventory/local/group_vars

* [terraform/openstack] Adds supplementary master groups

* [terraform/openstack] Updates documentation avoiding manual setups for bastion (as they are not needed now).

* [terraform/openstack] Supplementary master groups in docs.

* [terraform/openstack] Fixes repeated usage of master fips instead of bastion fips

* [terraform/openstack] Missing change for network_id to subnet_id

* [terraform/openstack] Changes conditional to element( concat ) form to avoid type issues with empty lists.