Use the same domain for all acceptance tests

[philippschulte]

The acceptance tests of fastly/resource_fastly_service_v1_syslog_test.go are currently failing:

=== RUN   TestAccFastlyServiceV1_syslog_basic
--- FAIL: TestAccFastlyServiceV1_syslog_basic (3.58s)
    testing.go:569: Step 0 error: errors during apply:

        Error: 400 - Bad Request:

            Title:  Bad request
            Detail: Domain 'notadomain1.com' is already taken by another customer

          on /var/folders/n4/ggm3f3vn1wbcql74h1xqt0nr0000gn/T/tf-test305549885/main.tf line 2:
          (source code not available)


=== RUN   TestAccFastlyServiceV1_syslog_formatVersion
--- FAIL: TestAccFastlyServiceV1_syslog_formatVersion (3.17s)
    testing.go:569: Step 0 error: errors during apply:

        Error: 400 - Bad Request:

            Title:  Bad request
            Detail: Domain 'notadomain1.com' is already taken by another customer

          on /var/folders/n4/ggm3f3vn1wbcql74h1xqt0nr0000gn/T/tf-test259495287/main.tf line 2:
          (source code not available)


=== RUN   TestAccFastlyServiceV1_syslog_useTls
--- FAIL: TestAccFastlyServiceV1_syslog_useTls (3.72s)
    testing.go:569: Step 0 error: errors during apply:

        Error: 400 - Bad Request:

            Title:  Bad request
            Detail: Domain 'notadomain1.com' is already taken by another customer

          on /var/folders/n4/ggm3f3vn1wbcql74h1xqt0nr0000gn/T/tf-test063435457/main.tf line 2:
          (source code not available)

This PR aligns the domain "fastly-test.tf-%s.com" to all acceptance tests. After this PR is merged all acceptance tests use the same domain so that it can be easily replaced if it is used by another user again.

[phamann]

LGTM