marvis should be usable w/o having pylxd installed

[lpirl]

Otherwise, e.g.,

• node/docker.py imports command_executor.docker
• command_executor.docker imports LXDCommandExecutor
• command_executor/lxd import pylxd

which then fails.

Solution is probably to clear command_executor/__init__.py and make dependent code import from the command_executor submodules directly.

I'll look into if nobody volunteers.