Allow GCP buckets in other regions

[KastenMike]

This PR lets you create buckets in specific regions. Currently default args are used (StorageClass: "MULTI_REGIONAL", Location: "US"); Location default is mentioned here, StorageClass through testing. Settings are passed via config.Region like S3 does in s3Config func.

Example test (placed inside a new dir+file mike/mike_test.go in my case):

func TestRegionalBucket(t *testing.T) {
	fmt.Println("TestRegionalBucket......")
	projectId := "projectId goes here"
	configJSON := "configJSON goes here"
	bucket := "mimckayd-euro"
	location := "europe-west8"
	providerType := objectstore.ProviderTypeGCS

	ctx := context.Background()
	pc := objectstore.ProviderConfig{
		Type:     providerType,
		Region:   location,
	}
	secret := &objectstore.Secret{}
	secret.Type = objectstore.SecretTypeGcpServiceAccountKey
	secret.Gcp = &objectstore.SecretGcp{
		ProjectID:  projectId,
		ServiceKey: configJSON,
	}
	p, err := objectstore.NewProvider(ctx, pc, secret)
	if err != nil {
		t.Errorf("NO NewProvider! %v", err)
	}

	b, err := p.CreateBucket(ctx, bucket)
	if err != nil {
		t.Errorf("NO BUCKET! %v", err)
	}

	fmt.Println("bucket created...", b.String())
}

I wanted to add some kind of test to objectstore_test like this:

func (s *ObjectStoreProviderSuite) TestGCSRegions(c *C) {
	if s.osType != ProviderTypeGCS {
		c.Skip("Test only applicable to GCS")
	}
	gcsRegions := []string{"us-west1", "europe-west8", "asia-south2"}
	ctx := context.Background()
	for _, region := range gcsRegions {
		s.initProvider(c, region)
		bucketName := s.createBucketName(c)
		_, err := s.provider.CreateBucket(ctx, bucketName)
		c.Check(err, IsNil)
		// try to find region from d~
		s.provider.DeleteBucket(ctx, bucketName)
	}
}

But at this point it isn't possible; it requires another update to stow to support it (exporting the real region), in particular: kastenhq/stow#27

Can be added in a future PR after that goes in

[KastenMike]

Alternatively config.StorageClass could be added but it would only be relevant for GCS

[ankitjain235]

LGTM.