autotest: lxc: factor out sudos in utils

Create a helper function to combine commands into a single `sudo bash -c` call.
This change is relanding a reverted attempt for the third time, with
still fewer moving parts and a feature flag.
Unlike previous attempts, this does not include any calls to the new
method.

BUG=chromium:809306
BUG=chromium:817569
BUG=chromium:821527
TEST=tryjob

Change-Id: Icbc1632ec9a9898fd234c7261421dda2ef10bc89
Reviewed-on: https://chromium-review.googlesource.com/963515
Commit-Ready: Jacob Kopczynski <jkop@chromium.org>
Tested-by: Jacob Kopczynski <jkop@chromium.org>
Reviewed-by: Jacob Kopczynski <jkop@chromium.org>
diff --git a/global_config.ini b/global_config.ini
index 04871cb..2bf4829 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -475,6 +475,7 @@
 
 [LXC_POOL]
 use_lxc_pool: False
+combine_sudos: False
 
 [SKYLAB]
 respect_static_labels: False