image/centos: umount filesystems under the image build directory
image/centos: umount filesystems under the image build directory

... before trying to remove that directory. No busy loopback devices
are hanging around after a failed build any more.

Closes-Bug: #1427170

Change-Id: I72a6e58f0501e73996964bc033556747b56c43bf

dancn/fuel-main-dev,SergK/fuel-main,ddepaoli3/fuel-main-dev,AnselZhangGit/fuel-main,AnselZhangGit/fuel-main,ddepaoli3/fuel-main-dev,teselkin/fuel-main,teselkin/fuel-main,huntxu/fuel-main,dancn/fuel-main-dev,eayunstack/fuel-main,Fiware/ops.Fuel-main-dev,Fiware/ops.Fuel-main-dev,Fiware/ops.Fuel-main-dev,dancn/fuel-main-dev,stackforge/fuel-main,teselkin/fuel-main,AnselZhangGit/fuel-main,huntxu/fuel-main,stackforge/fuel-main,ddepaoli3/fuel-main-dev,SmartInfrastructures/fuel-main-dev,ddepaoli3/fuel-main-dev,stackforge/fuel-main,SergK/fuel-main,SergK/fuel-main,eayunstack/fuel-main,Fiware/ops.Fuel-main-dev,SmartInfrastructures/fuel-main-dev,AnselZhangGit/fuel-main,huntxu/fuel-main,teselkin/fuel-main,SmartInfrastructures/fuel-main-dev,eayunstack/fuel-main,dancn/fuel-main-dev,SmartInfrastructures/fuel-main-dev