CentOS 8: separate kolla build tag from deploy tag

Prevously the container image tag applied to built images was configured
via 'kolla_openstack_release'. This variable also controlled the tag
used for deployed images. This could cause problems during the CentOS 8
transition, where we need to build two sets of images, and Kolla Ansible
may apply a '-centos8' suffix to the tag we specify on CentOS 8.

This change separates the tag applied to built images into a different
variable - 'kolla_tag'. The default is still 'kolla_openstack_release'.

Change-Id: I8e1d877ee91a07b86cb858d25b841f8bfcd50e21
Story: 2006574
Task: 39487