This can only happen if openshift_release is not a string. Using just openshift_release=3.10 will result in a float value, not a string. Unfortunately, we can't simply cast to string as the zero will get removed. You can set openshift_release="3.10" or openshift_release=v3.10.
Lets make sure that the example inventories show the value quoted and that any time we're printing out an error about openshift_release we make sure to make it clear there that it needs to be quoted.
https://github.com/openshift/openshift-ansible/pull/8486 in openshift-ansible-3.10.0-0.51.0
Verified this bug with openshift-ansible-3.10.0-0.51.0, and PASS. TASK [openshift_sanitize_inventory : Abort when openshift_release is invalid] *** Thursday 24 May 2018 22:16:25 -0400 (0:00:00.053) 0:00:02.726 ********** fatal: [host-8-246-83.host.centralci.eng.rdu2.redhat.com]: FAILED! => {"changed": false, "failed": true, "msg": "openshift_release is \"a3.10\" which is not a valid version string.\nPlease set openshift_release to a version string and ensure that the value is quoted, ex: openshift_release=\"3.4\"."} fatal: [host-8-247-24.host.centralci.eng.rdu2.redhat.com]: FAILED! => {"changed": false, "failed": true, "msg": "openshift_release is \"a3.10\" which is not a valid version string.\nPlease set openshift_release to a version string and ensure that the value is quoted, ex: openshift_release=\"3.4\"."}
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHBA-2018:1816