If we don't close connection to ceph cluster,
The connection count increases with cinder bakcup until exceed max open files.
Then rise OSError: [Errno 24] Too many open files
This patch is to close connection to ceph cluster after cinder backup.
Change-Id: I2fb243d2a57771dc3589e96db54e998e2c1c8ef7
Closes-bug: #1628626