MB-100 extract reusable part of test-groups.rb to a separate file

... to be able to use in other tests

Change-Id: I9ee3b601e71f293384a73e121de9d62d8296d9a6
Reviewed-on: http://review.couchbase.org/30461
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>