some ebucketmigrator_srv re-factoring

- unify building args under the same api
- get rid of erlang:apply in one place

Change-Id: Ie07348052ab37d36553645e989c8b499a980b9a8
Reviewed-on: http://review.couchbase.org/36883
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>