Fix variable typo of use_syslog in lib/tempest

TEMPEST_CONF is not set but used it. As a result the meaningless file
'DEFAULT' is created. So this commit fixes to use proper variable name.

Closes-bug: #1394969
Change-Id: If6e58f269319df3c4f71b4fc13aa855c63f7e3a3
Merge "Fix variable typo of use_syslog in lib/tempest"