firstboot: disable existing repos before pivot

On FCOS boot OS extensions are being installed, so in order to use
only shipped RPMs and avoid pulling in updates from public repos existing
repos need to be disabled

templates/common/_base/units/machine-config-daemon-firstboot.service.yaml

@@ -14,6 +14,8 @@ contents: |
  [Service]
  Type=oneshot
  RemainAfterExit=yes
+ # Disable existing repos (if any) so that OS extensions would use embedded RPMs only
+ ExecStartPre=-/usr/bin/sh -c "sed -i 's/enabled=1/enabled=0/' /etc/yum.repos.d/*.repo"
  ExecStart=/run/bin/machine-config-daemon firstboot-complete-machineconfig
 
  {{if .Proxy -}}