[2.5] MAAS doesn't set yum proxy for CentOS

[Andres Rodriguez]

During installation, MAAS doesn't set the maas_proxy to the centos configuration.

MAAS should support setting this, even thought cloud-init doesn't support it today.

A late_command could automatically be added by adding something like this:

 "echo proxy=<proxy> >> /etc/yum.conf"

Another thing that could be done:

    if [ ! -z "${http_proxy-}" ]; then
        if [ "$OS_NAME" = "centos" ]; then
            debug 1 "configuring proxy ${http_proxy}"
            inside "$name" sh -c "echo proxy=$http_proxy >> /etc/yum.conf"
            inside "$name" sed -i s/enabled=1/enabled=0/ \
                /etc/yum/pluginconf.d/fastestmirror.conf
        else
            debug 1 "do not know how to configure proxy on $OS_NAME"
        fi
    fi