made requested_orphan_count field mandatory and be a positive integer

part of pivotal tracker story https://www.pivotaltracker.com/story/show/79157380

- added new constraints to requested_orphan_count_field
- refactored tests and factories to comply with new requirements

OSRA-86 #resolve
OSRA-83 #resolve
Merge pull request #4 from PurityControl/requested_orphan_count

made requested_orphan_count field mandatory and be a positive integer