Ref Impl: Refactor primary.py and secondary.py to pull out common elements

[awwad]

Since these will be implemented separately and possibly very differently by OEMs, I'm slightly nervous about having a common module that they both employ, but otherwise, we're just dealing with lots of copied code, which is problematic.

Examples:

• Completely identical:
  • Metadata validation in:
  • primary.get_validated_target_info
  • secondary.get_validated_target_info
• Common pieces:
  • Higher level validation in:
  • primary.primary_update_cycle
  • secondary.fully_validate_metadata

I thought about the Primary class inheriting from the Secondary class and adding functionality, which makes a lot of sense in the abstract, but... we have a certain aversion to complex object-oriented code.

[awwad]

This issue was moved to uptane/uptane#14