The template that builds the etcd config uses the local variable etcd_interface which defaults to ansible_default_ipv4.interface to build the etcd configuration. So on your host that uses eth0 it tries to lookup the ip address of the other host based on ansible_eth0.ipv4.address, which doesn't exist and problems ensue.

TASK: [etcd | Validate permissions on peer certificate files] *****************
ok: [host1.example.com] => (item=/etc/etcd/ca.crt)
changed: [host1.example.com] => (item=/etc/etcd/peer.crt)
changed: [host1.example.com] => (item=/etc/etcd/peer.key)
ok: [host2.example.com] => (item=/etc/etcd/ca.crt)
changed: [host2.example.com] => (item=/etc/etcd/peer.crt)
changed: [host2.example.com] => (item=/etc/etcd/peer.key)

TASK: [etcd | Write etcd global config file] **********************************
fatal: [host1.example.com] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'dict object' has no attribute u'ansible_ens192'", 'failed': True}
fatal: [host1.example.com] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'dict object' has no attribute u'ansible_ens192'", 'failed': True}
fatal: [host2.example.com] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'dict object' has no attribute u'ansible_eth0'", 'failed': True}
fatal: [host2.example.com] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'dict object' has no attribute u'ansible_eth0'", 'failed': True}

I believe this is fixed by https://github.com/openshift/openshift-ansible/pull/437 which switches the behaviour to use hostnames rather than looking up the ip addresses.

I'm not entirely sure how to setup an environment that varies the interface names but testing between bare metal and a virtual machine may be one way. Ultimately testing that this doesn't cause regressions should be sufficient if setting up an environment with different interface names proves to be difficult.
We've had one customer verify that this addresses the issue for them.