Build JSON API changes

• Exit plugins can now fail the build. All exit plugins are run at the end of the build regardless of whether the build has failed. Exit plugin failures for plugins marked as required for a successful build would previously not cause the build to fail, but they now do.
• koji_promote: now accepts optional target parameter
• pull_base_image: this plugin is no longer added if missing
• tag_and_push: this plugin is no longer added if missing

Bug fixes

• docker build: fix logging so that it is properly decoded from JSON
• koji_promote: use component, not name, to determine the name-version-release for the Koji build.

Improvements

• add_filesystem: new plugin for creating base images via Koji
• koji_promote:
• store originating Koji task ID in Koji build metadata
• show upload progress in log output
• store resulting Koji build ID as OpenShift build label
• tag build in Koji if target provided