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: I1f3bae0a39f28412c99e5d04f4364611f2a5facb
Merge "Only add logging handlers if there currently aren't any"