Upgrades, implemented free space validator

* renamed checker to health_checker
* created new module before_upgrade_checker
  where we keep all our checkers
* added flexible free space calculation
  system which recognizes mount points
* simplifed upgrade manager now it has
  only general upgarde logic

Closes-bug: #1333350
blueprint upgrade-to-5-1
Change-Id: Ic382001101d0ef178a048fc4aad8100de2f89317
Merge "Upgrades, implemented free space validator"