Move most of the code under cmd/nfd-master and cmd/nfd-worker into new
packages pkg/nfd-master and pk/nfd-worker, respectively. Makes extending
unit tests to "main" functions easier.
Implement TLS client certificate authentication. It is enabled by
specifying --ca-file, --key-file and --cert-file, on both the nfd-master
and nfd-worker side. When enabled, nfd-master verifies that the client
(worker) presents a valid certificate signed by the root certificate
(--ca-file). In addition, nfd-master does authorization based on the Common Name
(CN) of the client certificate: CN must match the node name specified in
the labeling request. This ensures (assuming that the worker
certificates are correctly deployed) that nfd-worker is only able to label
the node it is running on, i.e. prevents it from labeling other nodes.
Add support for TLS authentication. When enabled, nfd-worker verifies
that nfd-master has a valid certificate, i.e. signed by the given root
certificate and its Common Name (CN) matches the DNS name of the
nfd-master service being used. TLS authentication is enabled by
specifying --key-file and --cert-file on nfd-master, and, --ca-file on
nfd-worker.
