Move bucket_bound_hostname code into helper

[IlyaFaer]

For now we have three places in which we're using bucket_bound_hostname. The code is almost the same in all three places, so it'll be good to union it in single helper:

python-storage/google/cloud/storage/blob.py

Lines 517 to 522 in 616f107

	if ":" in bucket_bound_hostname:
	api_access_endpoint = bucket_bound_hostname
	else:
	api_access_endpoint = "{scheme}://{bucket_bound_hostname}".format(
	scheme=scheme, bucket_bound_hostname=bucket_bound_hostname
	)

python-storage/google/cloud/storage/bucket.py

Lines 2531 to 2536 in 616f107

	if ":" in bucket_bound_hostname:
	api_access_endpoint = bucket_bound_hostname
	else:
	api_access_endpoint = "{scheme}://{bucket_bound_hostname}".format(
	scheme=scheme, bucket_bound_hostname=bucket_bound_hostname
	)

python-storage/google/cloud/storage/client.py

Lines 1014 to 1020 in 616f107

	if ":" in bucket_bound_hostname: # URL includes scheme
	url = bucket_bound_hostname
	else: # scheme is given separately
	url = "{scheme}://{host}/".format(
	scheme=scheme, host=bucket_bound_hostname
	)

[frankyn]

that sounds good, thanks for catching the improvement.