modules.lxc.running_systemd: use command -v not which

[jfindlay]

Fixes #30676.

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html

[cachedout]

I am a little confused by this. At least on my system which is a built-in and command is as well, so it begs the question of why would one work and the other not. Have you tested this in the scenario outlined in the original issue?

cc: @terminalmage Do you have any additional thoughts here?

[jfindlay]

@cachedout, all I have is that based on my experience, and the referenced POSIX standard, command is more portable than which.

[terminalmage]

@cachedout command is built into bash, while which is packaged separately and is not part of the lxc rootfs image provided by the download template for LXC (which allows you to download pre-built rootfs images for various distros).

% rpm -q which
which-2.20-7.el7.x86_64
% rpm -qf /usr/bin/command
bash-4.2.46-19.el7.x86_64