scripts/maintenance/clean-aws.sh: Skip $tag_file for unset --tag-file

[wking]

d541760 (#34) cleaned up some related handling, but missed this. Before this commit, an unset --tag-file would have resulted in a useless $tag_file creation which was never written to, and the "Deleting resources with the following tags:" line wouldn't actually print the actual tag.json content.

[wking]

The e2e-aws errors were more EIP exhaustion:

6 error(s) occurred:

* module.vpc.aws_eip.nat_eip[3]: 1 error(s) occurred:

* aws_eip.nat_eip.3: Error creating EIP: AddressLimitExceeded: The maximum number of addresses has been reached.
	status code: 400, request id: ba756648-01c2-4239-a487-13db78e8b366
* module.vpc.aws_eip.nat_eip[2]: 1 error(s) occurred:

* aws_eip.nat_eip.2: Error creating EIP: AddressLimitExceeded: The maximum number of addresses has been reached.
	status code: 400, request id: dd73f862-b686-48d8-bb9f-ac6bf279661e
* module.vpc.aws_eip.nat_eip[1]: 1 error(s) occurred:

* aws_eip.nat_eip.1: Error creating EIP: AddressLimitExceeded: The maximum number of addresses has been reached.
	status code: 400, request id: 56c868ab-52f8-49eb-9d6c-71cd83062530
* module.vpc.aws_eip.nat_eip[0]: 1 error(s) occurred:

* aws_eip.nat_eip.0: Error creating EIP: AddressLimitExceeded: The maximum number of addresses has been reached.
	status code: 400, request id: e9391e87-b457-4af9-b075-a0b722a6ce3b
* module.vpc.aws_eip.nat_eip[5]: 1 error(s) occurred:

* aws_eip.nat_eip.5: Error creating EIP: AddressLimitExceeded: The maximum number of addresses has been reached.
	status code: 400, request id: bc974d8c-9a4f-4738-91e6-99cfd2b31318
* module.vpc.aws_eip.nat_eip[4]: 1 error(s) occurred:

* aws_eip.nat_eip.4: Error creating EIP: AddressLimitExceeded: The maximum number of addresses has been reached.
	status code: 400, request id: 0fe2f93a-66c8-4dd6-a492-7b135fd5aaa7

I've got a script cleaning out leaked EIPs now, so we should be clear on that front.

/retest

[yifan-gu]

/lgtm

[wking]

The e2e-aws errors were:

3 error(s) occurred:

* module.etcd.aws_instance.etcd_node[1]: 1 error(s) occurred:

* aws_instance.etcd_node.1: InvalidInstanceID.NotFound: The instance ID 'i-0150d60f2649d1d87' does not exist
	status code: 400, request id: 05448e9c-6944-454f-af9e-37dc93401eef
* module.etcd.aws_instance.etcd_node[0]: 1 error(s) occurred:

* aws_instance.etcd_node.0: InvalidInstanceID.NotFound: The instance ID 'i-0786610ecd86ae9f4' does not exist
	status code: 400, request id: 87c64bee-6d6e-4470-a288-9a2265f25bb8
* module.etcd.aws_instance.etcd_node[2]: 1 error(s) occurred:

* aws_instance.etcd_node.2: InvalidInstanceID.NotFound: The instance ID 'i-0a5267f13adab3e69' does not exist
	status code: 400, request id: 04fedcf6-20c0-4061-a2b8-6d9a018bdee2

Perhaps the effect of overeager resource leak protection (although I don't see those IDs in the logs of my leak-pruning runs). Anyway, give it another shot:

/retest

[wking]

Got:

 * aws_route.to_nat_gw.3: Error finding route after creating it: Unable to find matching route for Route Table (rtb-0082a80962fc09824) and destination CIDR block (0.0.0.0/0).

despite the same test passing just before. Weird.

/retest