Implement kernel version detection

[marquiz]

Make it possible to advertise also other than simple 'true' values for
feature labels.

Add a new 'kernel' feature source, detecting the kernel version. The
kernel version is split into multiple labels in order to make this more
usable in label selectors. Kernel version in the format X.Y.Z-patch will
be presented as
node.alpha.kubernetes-incubator.io/nfd-kernel-version-full=X.Y.Z-patch
node.alpha.kubernetes-incubator.io/nfd-kernel-version-major=X
node.alpha.kubernetes-incubator.io/nfd-kernel-version-minor=Y
node.alpha.kubernetes-incubator.io/nfd-kernel-version-revision=Z

The '-full' label will always be avaiable. The other labels if these
components can be parsed from the kernel version number.

[pmossakx]

Verified #149 and #157 on generic CentOS:

[root@pmossakx-centos node-feature-discovery]# git log -1
commit 6ebb52937fc810b327e3ed3844848483eb97d9fa
Merge: 05a8162 9994063
Author: Piotr Mossakowski <piotrx.mossakowski@intel.com>
Date:   Wed Sep 26 07:30:05 2018 +0000

    Merge remote-tracking branch 'origin/feature/os-release' into feature/kernel-version

[root@pmossakx-centos node-feature-discovery]# uname -r
4.4.156-1.el7.elrepo.x86_64

[root@pmossakx-centos node-feature-discovery]# cat /etc/redhat-release
CentOS Linux release 7.5.1804 (Core)

[root@pmossakx-centos node-feature-discovery]# kubectl get node pmossakx-centos -o json | jq .metadata.labels
{
<output omitted>
  "node.alpha.kubernetes-incubator.io/nfd-kernel-version-full": "4.4.156-1.el7.elrepo.x86_64",
  "node.alpha.kubernetes-incubator.io/nfd-kernel-version-major": "4",
  "node.alpha.kubernetes-incubator.io/nfd-kernel-version-minor": "4",
  "node.alpha.kubernetes-incubator.io/nfd-kernel-version-revision": "156",
  "node.alpha.kubernetes-incubator.io/nfd-os-release-ID": "centos",
  "node.alpha.kubernetes-incubator.io/nfd-os-release-VERSION_ID": "7",
  "node.alpha.kubernetes-incubator.io/node-feature-discovery.version": "v0.2.0-10-g6ebb529"
}

and custom ubuntu kernel:

root@pmossakx-ubuntu:~/node-feature-discovery# git log -1
commit 27e6443e7eff65c8f275fb94bd87c0d3da30e9f4
Merge: 26cae36 089ca10
Author: Piotr Mossakowski <piotrx.mossakowski@intel.com>
Date:   Thu Sep 27 10:45:06 2018 +0100

    Merge remote-tracking branch 'origin/feature/os-release' into feature/kernel-version

root@pmossakx-ubuntu:~/node-feature-discovery# uname -r
4.18.0-pmossakx

root@pmossakx-ubuntu:~/node-feature-discovery# lsb_release -a
Distributor ID: Ubuntu
Description:    Ubuntu 18.04 LTS
Release:        18.04
Codename:       bionic

root@pmossakx-ubuntu:~/node-feature-discovery# kubectl get node pmossakx-ubuntu -o json | jq .metadata.labels
{
<output omitted>
  "node.alpha.kubernetes-incubator.io/nfd-kernel-version-full": "4.18.0-pmossakx",
  "node.alpha.kubernetes-incubator.io/nfd-kernel-version-major": "4",
  "node.alpha.kubernetes-incubator.io/nfd-kernel-version-minor": "18",
  "node.alpha.kubernetes-incubator.io/nfd-kernel-version-revision": "0",
  "node.alpha.kubernetes-incubator.io/nfd-os-release-ID": "ubuntu",
  "node.alpha.kubernetes-incubator.io/nfd-os-release-VERSION_ID": "18.04",
  "node.alpha.kubernetes-incubator.io/node-feature-discovery.version": "v0.2.0-15-g27e6443-dirty"
}

[marquiz]

Based on discussion in #140, I changed the label names so that there now is a dot (instead of dash) between version and the version component, i.e. we would now have:

nfd-kernel-version.full=X.Y.Z-patch
nfd-kernel-version.major=X
nfd-kernel-version.minor=Y
nfd-kernel-version.revision=Z

[lmdaly]

Again LGTM with new naming convention!

[k8s-ci-robot]

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