backup: set default swiftclient log level to WARN
backup: set default swiftclient log level to WARN

Similar to change I0857cecd7d8ab0ee7e3e9bd6e15f4987ede4d653, the
swift backup driver cannot be logging stuff while in a thread during
read and write operations. swiftclient logs requests and responses
at DEBUG level, which can cause a thread switch and break the backup
operation.

This sets a default log level of WARN for swiftclient for the
cinder-backup service.

Change-Id: I67ac11276715290839c39fc1d0e333c61a38c461
Closes-Bug: #1745168

openstack/cinder,Datera/cinder,openstack/cinder,phenoxim/cinder,mahak/cinder,phenoxim/cinder,Datera/cinder,j-griffith/cinder,mahak/cinder,j-griffith/cinder