Implement new 'system' feature source. It now detects OS release
information from the os-release file, assumed to be available at
/host-etc/os-release. It currently creates two labels (assuming that the
corresponding fields are found in the os-release file), with example
values:
feature.node.kubernetes.io/system-os_release.ID=opensuse
feature.node.kubernetes.io/system-os_release.VERSION_ID=42.3
Also, update the template spec to mount /etc/os-release file from the
host inside the container.
