This commit fixes the bug in #2232 where cadvisor was not able to detect
the cloud provider if it's running on a custom AMI derived from
Amazon Linux 2.
It does so by checking /etc/os-release. However, from what I've read,
/etc/os-release is pretty much a systemd thing. Although Amazon Linux 2
comes with systemd, cadvisor cannot assume the existence of systemd in
other AMIs / OSes, therefore we would only be checking for
/etc/os-release if all other methods fail us.
