Use init() to handle plugin invocation

[notnoop]

Currently, nomad "plugin" processes (e.g. executor, logmon, docker_logger) are started as CLI
commands to be handled by command CLI framework. Plugin launchers use
discover.NomadBinary() to identify the binary and start it.

This has few downsides: The trivial one is that when running tests, one
must re-compile the nomad binary as the tests need to invoke the nomad
executable to start plugin. This is frequently overlooked, resulting in
puzzlement.

The more significant issue with executor in particular is in relation
to external driver:

• Plugin must identify the path of invoking nomad binary, which is not
  trivial; discvoer.NomadBinary() now returns the path to the plugin
  rather than to nomad, preventing external drivers from launching
  executors.

• The external driver may get a different version of executor than it
  expects (specially if we make a binary incompatible change in future).

This commit addresses both downside by having the plugin invocation
handling through an init() call, similar to how libcontainer init
handler is done in [1] and recommened by libcontainer [2]. init()
will be invoked and handled properly in tests and external drivers.

For external drivers, this change will cause external drivers to launch
the executor that's compiled against.

There a are a couple of downsides to this approach:

• These specific packages (i.e executor, logmon, and dockerlog) need to
  be careful in use of init(), package initializers. Must avoid having
  command execution rely on any other init in the package. I prefixed
  files with z_ (golang processes files in lexical order), but ensured
  we don't depend on order.
• The command handling is spread in multiple packages making it a bit
  less obvious how plugin starts are handled.

[1] drivers/shared/executor/libcontainer_nsenter_linux.go
[2] https://github.com/opencontainers/runc/tree/eb4aeed24ffbf8e2d740fafea39d91faa0ee84d0/libcontainer#using-libcontainer

[nickethier]

This LGTM! Can you add comments above the inits about what they are and why they are there. The PR description explains this well, it would be good to have it in the source for future devs.

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