Ignore pylint error 'hashlib' has no shaxxx member

The following pylint error was found:

cinder/backup/drivers/swift.py:486: [E1101, SwiftBackupDriver.backup]
Module 'hashlib' has no 'sha256' member

Affected patch:
https://review.openstack.org/#/c/110068/

A similar pylint error "Module 'hashlib' has no 'sha224' member"
was also generated.

However, both 'sha256' and 'sha224' are functions in 'hashlib'.  So
these errors should be ignored.

Change-Id: I84690813b6c6a15e661e27acdf97149996951978
Merge "Ignore pylint error 'hashlib' has no shaxxx member"