cryptic 'count' cannot be computed for ignition

[tomdavidson]

What keywords did you search in tectonic-installer issues before filing this one?

Various selections from the quoted error message below.

Is this a BUG REPORT or FEATURE REQUEST?

BUG REPORT / SUPPORT

Versions

Tectonic version (release or commit hash):

1.8.7-tectonic.1

Terraform version (terraform version):

0.10.8

Platform (aws|azure|openstack|metal|vmware):

AWS

What happened?

I'm using the installer as a module in my plan. Select files in this [gist](https://gist.github.com/tomdavidson/3a283449b1421c8b61c02345d07d1e68)
I set env, project_name, primary_region, the credentials - everything else are the defaults from the config.tf 

`terraform plan`  gives me:

* module.tectonic-installer.module.ignition_masters.data.ignition_file.etcd_peer_crt: data.ignition_file.etcd_peer_crt: value of 'count' cannot be computed
* module.tectonic-installer.module.ignition_masters.data.ignition_file.etcd_peer_key: data.ignition_file.etcd_peer_key: value of 'count' cannot be computed
* module.tectonic-installer.module.ignition_masters.data.template_file.advertise_client_urls: data.template_file.advertise_client_urls: value of 'count' cannot be computed
* module.tectonic-installer.module.etcd.data.ignition_systemd_unit.locksmithd: data.ignition_systemd_unit.locksmithd: value of 'count' cannot be computed
* module.tectonic-installer.module.ignition_masters.data.template_file.initial_advertise_peer_urls: data.template_file.initial_advertise_peer_urls: value of 'count' cannot be computed
* module.tectonic-installer.module.ignition_masters.data.ignition_file.etcd_server_key: data.ignition_file.etcd_server_key: value of 'count' cannot be computed
* module.tectonic-installer.module.ignition_masters.data.ignition_file.etcd_server_crt: data.ignition_file.etcd_server_crt: value of 'count' cannot be computed
* module.tectonic-installer.module.ignition_masters.data.template_file.etcd_names: data.template_file.etcd_names: value of 'count' cannot be computed
* module.tectonic-installer.module.ignition_masters.data.ignition_file.etcd_ca: data.ignition_file.etcd_ca: value of 'count' cannot be computed

References

#1966 looks similar but I am not using an existing VPC.