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
handli...