Only add logging handlers if there currently aren't any

This corrects an odd problem where Horizon would stand up multiple
client objects, which would cause duplicate/triplicate/dozens of
repeated log lines in its log files, due to multiple identical
handlers being added to the logging object

Fixes Bug 1182678

Change-Id: I020b3999c9008b996286ccb74a7801c96c43e71c
Merge "Only add logging handlers if there currently aren't any"