Specify log file name for Nova API

When only the directory is specified, separate log files
are created for the Nova API / metadata services with a
-wsgi postfix. This affects the 'programname' field in
Fluentd which affects the processing of these logs. This
is a regression.

When the log file name is specified, the directory is
not required.

Closes-Bug: #1950185
Change-Id: I8fec8b787349f83c05d8af7f52fc58da7c3e9cc4