s3logging: default S3 domain to s3.amazonaws.com to match api default

[catsby]

Set default value for the domain attribute in the s3logging block. Right now if it's not in us-east-1, you need to specify a value, but it seems now that the API will provide a default of s3.amazonaws.com so we need to match it.

Users specifying any valid value are unaffected, and users without a value are seeing a perpetual diff, so this should be fine in terms of backwards compatibility regarding the struct hash.

All our existing tests supplied a value here, so I add a new test with that uses the default.

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./fastly -v -run=TestAccFastlyServiceV1_s3logging -timeout 120m
=== RUN   TestAccFastlyServiceV1_s3logging_basic
--- PASS: TestAccFastlyServiceV1_s3logging_basic (33.43s)
=== RUN   TestAccFastlyServiceV1_s3logging_domain_default
--- PASS: TestAccFastlyServiceV1_s3logging_domain_default (15.20s)
=== RUN   TestAccFastlyServiceV1_s3logging_s3_env
--- PASS: TestAccFastlyServiceV1_s3logging_s3_env (16.60s)
=== RUN   TestAccFastlyServiceV1_s3logging_formatVersion
--- PASS: TestAccFastlyServiceV1_s3logging_formatVersion (14.53s)
PASS
ok      github.com/terraform-providers/terraform-provider-fastly/fastly 79.774s

Should fix #10 /cc @DanielRussell

[radeksimko]

I left you one low-prio comment. If I understand correctly the API's behaviour regarding defaults has changed already, which already caused diffs to some people, so there's no point of adding any state migration. So if that's the case this LGTM.

[radeksimko]

I guess we could also check that new default value of s3logging.<idx>.domain here, but it's no big deal.