MB-100: fix OSError: [Errno 36] File name too long

http://qa.hq.northscale.net/view/2.0.1/job/windows-64-2.0-upgrade/6/consoleFull

it happens because max len for '%y-%b-%d_%H-%M-%S' is 18

Change-Id: I676a92fbc4666eb94bac3bd7afb8be2226c7f6dc
Reviewed-on: http://review.couchbase.org/24850
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>