Investigated a bit.

On a clean 4.2.8 allo repos in ovirt* are enabled but with the following directive:

# imgbased: set-enabled

includepkgs = ovirt-node-ng-image-update ovirt-node-ng-image ovirt-engine-appliance

installing ovirt-release43.rpm doesn't apply above directives because they're added there by ovirt-release-host-node post installation script.

So, if you install ovirt-release43, then you need to:

yum update ovirt-release-host-node; yum update;

to finish the node upgrade.

So the most clean way to upgrade from 4.2.8 to 4.3 is  "yum install https://resources.ovirt.org/pub/ovirt-4.3/rpm/el7/noarch/ovirt-node-ng-image-update-4.3.0-1.el7.noarch.rpm "

this leave ovirt-4.2 repo files in /etc/yum.repos.d ; after rebooting into 4.3 layer it's safe to remove them