* client/executor: refactor client to remove interpolation

* executor: POC libcontainer based executor

* vendor: use hashicorp libcontainer fork

* vendor: add libcontainer/nsenter dep

* executor: updated executor interface to simplify operations

* executor: implement logging pipe

* logmon: new logmon plugin to manage task logs

* driver/executor: use logmon for log management

* executor: fix tests and windows build

* executor: fix logging key names

* executor: fix test failures

* executor: add config field to toggle between using libcontainer and standard executors

* logmon: use discover utility to discover nomad executable

* executor: only call libcontainer-shim on main in linux

* logmon: use seperate path configs for stdout/stderr fifos

* executor: windows fixes

* executor: created reusable pid stats collection utility that can be used in an executor

* executor: update fifo.Open calls

* executor: fix build

* remove executor from docker driver

* executor: Shutdown func to kill and cleanup executor and its children

* executor: move linux specific universal executor funcs to seperate file

* move logmon initialization to a task runner hook

* client: doc fixes and renaming from code review


* taskrunner: use shared config struct for logmon fifo fields

* taskrunner: logmon only needs to be started once per task