enable autogenerating token (#375)

* Generate token

If a token is not explicitly provided, let the first server generate a
random one. Such a token is saved on the first server and the playbook
can retrieve it from there and store it a a fact. All other servers and
agents can use that token later to join the cluster. It will be saved
into their environment file as usual.

Signed-off-by: Marko Vukovic <8951449+anon-software@users.noreply.github.com>

* Document that token is (mostly) optional now

The token is still required when using Vagrant.

Signed-off-by: Marko Vukovic <8951449+anon-software@users.noreply.github.com>