aws_efs_mount_target.mount_target: value of 'count' cannot be computed

[mgeerdsen]

When initially trying to set up the infrastructure from https://github.com/wellcometrust/workflow/tree/new-infra I ran into the following error:

module.goobi.module.efs.aws_efs_mount_target.mount_target: aws_efs_mount_target.mount_target: value of 'count' cannot be computed

It appears that count cannot be computed in the EFS module as it is derived via length() from var.subnets which have not yet been created.

Similiar issues have been described in hashicorp/terraform#10857

I will add a PR in a bit.