Use current CentOS when building ISO
Use current CentOS when building ISO

Switch make system to use latest available CentOS version, which is always
accessible at http://mirror.centos.org/centos/$release/.

Previous releases could be used if needed be, for example:
make MIRROR_CENTOS=http://vault.centos.org/7.1.1503/

Change-Id: Id15ddf90fdf5222f94d14865a42680ce8b955eef

SergK/fuel-main,stackforge/fuel-main,stackforge/fuel-main,SergK/fuel-main,SergK/fuel-main,stackforge/fuel-main