Add configuration for not mounting /tmp

[KexyBiscuit]

Rationale

When there's not plenty of RAM, which is common scenario, ciel build will fail due to no space left on device (/tmp). It would be great if there's a configuration for disabling the mount.

[liushuyu]

Should have been fixed in 9beafab.

Now systemd-nspawn should not mount /tmp as a tmpfs mount-point. (Requires a somewhat recent systemd to work).