Fix various issues found by testing Freshmaker with real depending services:
    
    - When FRESHMAKER_CONFIG_FILE is defined, load it even when using DEVELOPER_ENV.
    - Get the git branch name from the Brew build instead of setting it to "unknown"
      every time.
    - Set container image release to XX.timestamp instead of XX.YY.timestamp.
    - Pass models.py Event instance to prepare_yum_repo instead of events.py Event
      instance.
    - Remove call of non-existing _build_first_batch method in Errata handler.
    - Pass full config file to Koji.
    - Fix bug when Koji build was always 'scratch' build.
    - Do not include ".eng" in topic suffix for handler, this should be part of
      TOPIC_PREFIX configuration.