[autotest] Move get_global_afe_hostname to server/site_utils

Move get_global_afe_hostname to server/site_utils so that
other code can import it without django env setup hassle.

TEST=Run unittest, run shard_client
BUG=chromium:568873

Change-Id: I574d3dc00e2f79af0283487ec9cac6fb8bfd9fd4
Reviewed-on: https://chromium-review.googlesource.com/317276
Commit-Ready: Fang Deng <fdeng@chromium.org>
Tested-by: Fang Deng <fdeng@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
4 files changed