Generate bucket names and object names per test instead of once at the
top level. Should help avoid failures like this one:
https://travis-ci.org/hashicorp/terraform/jobs/100254008
All storage tests checked on this commit:
```
TF_ACC=1 go test -v ./builtin/providers/google -run TestAccGoogleStorage
=== RUN TestAccGoogleStorageBucketAcl_basic
--- PASS: TestAccGoogleStorageBucketAcl_basic (8.90s)
=== RUN TestAccGoogleStorageBucketAcl_upgrade
--- PASS: TestAccGoogleStorageBucketAcl_upgrade (14.18s)
=== RUN TestAccGoogleStorageBucketAcl_downgrade
--- PASS: TestAccGoogleStorageBucketAcl_downgrade (12.83s)
=== RUN TestAccGoogleStorageBucketAcl_predefined
--- PASS: TestAccGoogleStorageBucketAcl_predefined (4.51s)
=== RUN TestAccGoogleStorageObject_basic
--- PASS: TestAccGoogleStorageObject_basic (3.77s)
=== RUN TestAccGoogleStorageObjectAcl_basic
--- PASS: TestAccGoogleStorageObjectAcl_basic (4.85s)
=== RUN TestAccGoogleStorageObjectAcl_upgrade
--- PASS: TestAccGoogleStorageObjectAcl_upgrade (7.68s)
=== RUN TestAccGoogleStorageObjectAcl_downgrade
--- PASS: TestAccGoogleStorageObjectAcl_downgrade (7.37s)
=== RUN TestAccGoogleStorageObjectAcl_predefined
--- PASS: TestAccGoogleStorageObjectAcl_predefined (4.16s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/google 68.275s
```