Upgrades, fix error message formatting
Upgrades, fix error message formatting

In prepare_upgrade_files function was
wrong strings concatination. As result
user saw bash errors when he run the
script.

Closes-bug: #1337937
blueprint upgrade-to-5-1

Change-Id: Iaf734aac537f07c10811a382cbe90c2fe7f8f393

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