[autotest] Pass current user into container for RPC usage

RPC is called using current OS user if not specified (frontend.py). This causes
an issue for autoserv running inside container, as the OS user is always root.
The root user may fail check_for_acl_violation_hosts if a host has special ACL
that does not allow `everyone`.

This CL add an SSP/user config in global config, lxc related code is updated so
that the OS user of autoserv running in host will be passed into the container
through shadow_config.ini. And frontend will use such user to make RPC.

BUG=chromium:480525
TEST=local
sudo python site_utils/lxc_functional_test.py -v -s
attach to the container
update SSP/user with a user name (existing one should be root as the process
site_utils/lxc_functional_test.py is executed with sudo)
update frontend.py with code in this CL (this means only newer builds won't
be hit by this bug.)
run some RPC:
import common
from autotest_lib.server.cros.dynamic_suite import frontend_wrappers
_AFE = frontend_wrappers.RetryingAFE()
print _AFE.run('get_stable_version')

Also make sure afe in the host still works.

Change-Id: I917d832769a8e44d46f4fcaaf67b899d49715910
Reviewed-on: https://chromium-review.googlesource.com/277351
Trybot-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Fang Deng <fdeng@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
3 files changed