[Review] Request from 'digitaltom' @ 'SUSE/connect/review_140527_upgrade_product_support'

Please review the following changes:
  * 498c2aa fixes for rubocop
  * 27bc406 add product_upgrade method for yast, update methods docs
  * 951d4c3 adding upgrade product method to Connect::Client
  * a17eed5 fix parameter name: it's product_id rigt now
  * 1f0c18f Merge pull request #80 from SUSE/review_140526_include_global_declaration_for_root_parameter
  * 38bb9cf Merge pull request #81 from SUSE/review_140526_secure_yast_from_not_calling_zypper
  * 065e1d1 reset filesystem_root to nil
  * 5c9766b fix whitespace
  * de4bf8e fix indentation
  * a012490 do less stubbing in yast tests
  * 62fe5f3 don't query zypper for the base product on class initialization
  * f9e864e setting filesystem root to nil by default
  * e438228 using after filter
  * 8f268ee fix string checks in feature tests
  * c82a07e using a class instance variable for the root parameter SUSE::Connect::System.filesystem_root
  * 25ec49b Move global declaration to one place where it will be required, and ensure that is required during tests