unconfigured clients look in the [clients] section for defaults

This change enables clients that have no specific settings in
the [clients_xxx] sections of the configuration file to get defaults
from the [clients] section.

Change-Id: I071bd77a2e1f0ad366b80c095917a8debc5cef2b
Closes-Bug: 1379958
(cherry picked from commit 4fd5087499b0f3dae16b183eba825e642efb0e95)
Merge "unconfigured clients look in the [clients] section for defaults" into stable/juno