Refactor yum.py collecting packages functionality

Since yum.py functions to get desired packages from dependencies and from instances are quite similar - they need to be refactored to run common part only once. Generally, both function get list of requirements, convert them to rpm names and return list. This is what is very similar about them.
Moreover, it is required that both functions return Requirements objects instead of objects and strings to prevent additional checking and conversion before passing them to install transaction.
Possibly, convert_names_to_rpm should be able to work with Requirement objects, but this should be checked.