Initialize devicemapper in NewRuntimeFromDIrectory
devmapper: create device nodes 'on create' instead of 'on resume'
Merge branch 'dm-refactor-init' into dm

Conflicts:
	runtime_test.go
WIP: debugging dm-base-hash + dm-refactor-init
Hack: don't run integration tests in /var/lib/docker/unit-tests; add missing cleanups in a few tests
Change default values for devicemapper as variable instead of env
Hack: fix tests which didn't cleanup properly
Add debug messages while testing devicemapper