allow keeping of existing loggers with fileConfig

Currently, common.log.py setup() will disable all existing
loggers that were set up before importing a custom logging
configuration from file. This patch deprecates option name
'log-config' in favor of 'log-config-append' and makes
disable_existing_loggers with default value False.

Closes-Bug: #1169328
Closes-Bug: #1238349
DocImpact

Change-Id: I1f25300d44b04ca5bcdd9b505319f0d089a9c964
Merge "allow keeping of existing loggers with fileConfig"