Fixes#7433
When an EFS File System is created via Terraform, Deleted from the AWS
console, then Terraform would give us as error as:
```
* aws_efs_file_system.file_system: FileSystemNotFound: File system
'fs-9d739e54' does not exist.
status code: 404, request id:
d505a682-3ec7-11e6-81d3-1d41202f0881
```
On a 404, we now remove the EFS File System from state so that Terraform
can recreate it as expected
Paul Stack