CONLOGGER_LEVEL should exit if already configured

This seemed to have already been the intention in the code, but we
shouldn't override this if already present.

Change-Id: I751c227949a71d8424c1aaa33b1c83360602b3eb
Reviewed-on: http://review.couchbase.org/42168
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>