Currently we use CentOS 8 nodepool images for all CI build jobs. This
means we can't use the ansible_distribution_major_version fact to detect
whether we are building CentOS 7 or 8 images. Due to this, all CentOS
images (including CentOS 7) are currently published with a
'master-centos8' tag.
This change fixes the issue by using the base_tag variable which is
defined for CentOS 8 build jobs.
Change-Id: I29b7b8ee742120107a11bd419ac58ab5c09b0b75
Partially-Implements: blueprint centos-rhel-8