MultipartRequestInputFactory

Note: Factory class produces MultipartRequestInput object
for ConnectinAdapter.multipartRequest method. Class should
help a newbie to understand relationship between MultipartType
and set MultipartRequestBody children.

* factory class with an input validation
* test suite for factory MultipartRequestInput

Change-Id: Id728fa3e6f15cf8c83442a23b0b118fd35f2c5ee
Signed-off-by: Vaclav Demcak <vdemcak@cisco.com>