Updated CentOS 6 dockerfile to properly install devtoolset-7.
Updated CentOS 6 dockerfile to properly install devtoolset-7.

The previously used yum command was not able to properly install
the required `devtoolset-7` package since the containing repository
was only visible after the first `yum` invocation finished.

This would result in `yum` silently ignoring the package, producing
a broken CentOS 6 docker image.

Review: https://reviews.apache.org/r/71329

Gilbert88/mesos,Gilbert88/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,kaysoky/mesos,abudnik/mesos,Gilbert88/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,kaysoky/mesos,abudnik/mesos,kaysoky/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos