Compare Fuel versions as well as MOS ones

* Add rspec test cases for tricky and usual version formats, e.g.
  tricky: '2014.1.fuel5.0-mira4' vs '2014.1.1.fuel5.1-mira0'
  usual: '2014.1.fuel5.0-mira4' vs '2014.1.fuel5.1-mira0'
  (both for downgrading and installing cases)
* Make a yum downgrade decision based BOTH onto MOS versions and
  Fuel versions comparsion results instead of full MOSt versions
  ('fuel' hardcoded as a separator for MOS and Fuel versions in MOSt)

Closes-bug: #1342041

Change-Id: Icacec5ed01510ab526a4b0e6092efdf203a6a2cb
Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>