Add options ipc_mode, pid_mode and uts_mode to docker driver.

May only be used if priviliged mode is enabled in driver config.

Improve nomad status on periodic jobs

Add ProhibitOverlap option to PeriodicConfig

Qemu driver uses BasicExecutor

Allow lookups based on short identifiers

* Fix tests
* Update documentation

Improved update docs

Adding a , between args to netdav

Include gocov stuff in tools

Mount task and alloc dirs in the rkt driver

These two tools are used in the "cov" Makefile target, but installing
these were not automated. This PR removes the ambiguity where a
developer tries to do a `make cov` and finds that the tools are not
installed.