When running the installer with an install config that contains a network name but no VNic profile, and on the oVirt side there is more then one VNIC profile attached to the network then the installer will fail with error on missing tfvars.
The missing tfvars are all the oVirt specific vars and the tfvarts.json file is never written.

Example of the error:
```
....

ERROR Error: No value for required variable        
ERROR                                              
ERROR   on ../../../openshift-install-948796804/variables-ovirt.tf line 111: 
ERROR  111: variable "ovirt_master_affinity_groups" { 
ERROR                                              
ERROR The root module input variable "ovirt_master_affinity_groups" is not set, and 
ERROR has no default value. Use a -var or -var-file command line argument to provide 
ERROR a value for this variable.                   
ERROR                                              
ERROR Failed to read tfstate: open /tmp/openshift-install-948796804/terraform.tfstate: no such file or directory 
FATAL failed to fetch Cluster: failed to generate asset "Cluster": failed to create cluster: failed to apply Terraform: failed to complete the change 
```

When looking into the code we do check that and should return a good error message[1], but since this is a custom error and the err var is actually nil then we return with nil which causes the installation to continue and fail when it can't start terraform.

How to test:
Try to install openshift on oVirt with a network that has 2 VNICs attached and expect to get a clear error message:

"failed to compute values for Engine platform, there are multiple vNIC profiles. found 2 vNIC profiles for network _______"

[1] https://github.com/openshift/installer/blob/master/pkg/asset/cluster/tfvars.go#L534