fix to get OS name from /etc/os-release
fix to get OS name from /etc/os-release

1)unable to retrive OS flavour from /etc/issue in CentOS
2)made changes to the script to get OS name from /etc/os-release
Closes-Bug: #1538942

Change-Id: I40e02f11dbd5e9a391f46876883df0221088a3d5

Juniper/contrail-fabric-utils,Juniper/contrail-fabric-utils