For the CentOS 7 to 8 transition, we will have a period where both
CentOS 7 and 8 images are available. We differentiate these images via a
tag - the CentOS 8 images will have a tag of train-centos8 (or
master-centos8 temporarily).

To achieve this, and maintain backwards compatibility for the
openstack_release variable, we introduce a new 'openstack_tag' variable.
This variable is based on openstack_release, but has a suffix of
'openstack_tag_suffix', which is empty except on CentOS 8 where it has a
value of '-centos8'.

Change-Id: I12ce4661afb3c255136cdc1aabe7cbd25560d625
Partially-Implements: blueprint centos-rhel-8