MB-100: fixed dialyzer found bug

It's obvious that misc:get_env_default is not for os:getenv but for
application:get_env. I don't know what I was thinking.

Change-Id: I667922935ec2be66033640e51745f36bf1aec4bc
Reviewed-on: http://review.couchbase.org/25809
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>