[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>
diff --git a/server/site_utils.py b/server/site_utils.py
index 6d44467..52ab791 100644
--- a/server/site_utils.py
+++ b/server/site_utils.py
@@ -479,6 +479,12 @@
     return bool(hostname)
 
 
+def get_global_afe_hostname():
+    """Read the hostname of the global AFE from the global configuration."""
+    return global_config.global_config.get_config_value(
+            'SERVER', 'global_afe_hostname')
+
+
 def is_restricted_user(username):
     """Determines if a user is in a restricted group.