[autotest] Fix path in scheduler/shard/common.py

The sharding code has been moved to scheduler/shard, therefore
common.py needs to go 2 directory levels up to get to the correct
autotest_dir.

BUG=None
TEST=Ran suites.
DEPLOY=shard_client

Change-Id: I302d138c672d8d9e75a70ab235deb087ebf688bc
Reviewed-on: https://chromium-review.googlesource.com/221815
Reviewed-by: Fang Deng <fdeng@chromium.org>
Commit-Queue: Jakob Jülich <jakobjuelich@chromium.org>
Tested-by: Jakob Jülich <jakobjuelich@chromium.org>
3 files changed