Flexible messaging.
    
    Summary:
    This replaces the fedmsg message publication with a plugin-based approach.
    
    Plugins can be specified using setuptools entry-points and must extend an
    abstract base class defined in `resultsdb.messaging`.
    
    The original fedmsg publication code is reimplemented as a new plugin and a new
    `DummyPlugin` is added for use in the test suite.  The original test cases all
    still pass. Two new test cases are added that use and verify the DummyPlugin.
    
    Test Plan: Run the functional test suite.
    
    Reviewers: tflink, jskladan
    
    Reviewed By: jskladan
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1061