Mark methods used in class only with prefix "_"
Mark methods used in class only with prefix "_"

In the swift driver the three methods prepare_nbackup, backup_chunk
and finalize_backup are used internally in the class only and are
not meant to be used from outside. Thus marked with prefix "_"

Change-Id: Idf3a75f2e799c16b1442a349e0724d3e5af2795a

inkerra/cinder,abusse/cinder,winndows/cinder,bswartz/cinder,eharney/cinder,Hybrid-Cloud/cinder,alex8866/cinder,scality/cinder,Akrog/cinder,bswartz/cinder,apporc/cinder,tlakshman26/cinder-https-changes,Accelerite/cinder,maelnor/cinder,takeshineshiro/cinder,nexusriot/cinder,Datera/cinder,dims/cinder,saeki-masaki/cinder,duhzecca/cinder,potsmaster/cinder,takeshineshiro/cinder,julianwang/cinder,cloudbase/cinder,scality/cinder,cloudbau/cinder,nikesh-mahalka/cinder,sasukeh/cinder,CloudServer/cinder,saeki-masaki/cinder,NeCTAR-RC/cinder,ntt-sic/cinder,blueboxgroup/cinder,sasukeh/cinder,NetApp/cinder,inkerra/cinder,JioCloud/cinder,Paul-Ezell/cinder-1,scottdangelo/RemoveVolumeMangerLocks,tlakshman26/cinder-new-branch,Paul-Ezell/cinder-1,abusse/cinder,ge0rgi/cinder,julianwang/cinder,apporc/cinder,spring-week-topos/cinder-week,NetApp/cinder,openstack/cinder,Datera/cinder,NeCTAR-RC/cinder,hguemar/cinder,redhat-openstack/cinder,openstack/cinder,petrutlucian94/cinder,spring-week-topos/cinder-week,hguemar/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,eharney/cinder,tlakshman26/cinder-new-branch,j-griffith/cinder,potsmaster/cinder,rickerc/cinder_audit,Akrog/cinder,blueboxgroup/cinder,Hybrid-Cloud/cinder,rickerc/cinder_audit,github-borat/cinder,scottdangelo/RemoveVolumeMangerLocks,Accelerite/cinder,Thingee/cinder,CloudServer/cinder,leilihh/cinder,mahak/cinder,rakeshmi/cinder,leilihh/cinder,github-borat/cinder,nexusriot/cinder,petrutlucian94/cinder,maelnor/cinder,nikesh-mahalka/cinder,alex8866/cinder,cloudbau/cinder,j-griffith/cinder,tlakshman26/cinder-https-changes,manojhirway/ExistingImagesOnNFS,Nexenta/cinder,duhzecca/cinder,winndows/cinder,rakeshmi/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,JioCloud/cinder,cloudbase/cinder,dims/cinder,Thingee/cinder,Nexenta/cinder,manojhirway/ExistingImagesOnNFS,redhat-openstack/cinder,mahak/cinder,phenoxim/cinder,phenoxim/cinder,ntt-sic/cinder,Thingee/cinder