handle missing account properties for GerritChangeSource

* create a separate function to convert account properties
* make use of the function in all cases where the situation might take place
* update test cases

Fixes ticket:2918
update release notes
address pep8 errors
Merge branch 'bug-2918' of github.com:sa2ajj/buildbot into pr1241
_gerrit_user_to_author: fix codestyle to use string manipulation

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>