Fix perms to just set exec bit

Update cluster tutorial to generate random client UUIDs

Make example clients generate random HostIDs

The tutorial has you make two clients on the same Vagrant box, so they
end up with the same secret ID.

Fixes #2661.

Add SecurityOpt as a config field in Docker driver

New Update block syntax

The way the copying was happening on the alloc_runner was by temporarily
setting the alloc.Job to nil, copying and then restoring it. This
created an issue in which when the alloc was shared (which it is in
server/client mode and between alloc_runner/task_runner) there were race
conditions that could create a panic.

Fixes https://github.com/hashicorp/nomad/issues/2605