openqa-onebox: tweak db host stuff
    
    Using the machine's own hostname works for the ansible delegate
    stuff but doesn't work for openQA itself (if you try and access
    the DB by hostname like this, postgres denies access; you have
    to use 'localhost' for postgres to allow it). Using 'localhost'
    works for postgres but doesn't do the right thing for delegation.
    Let's use 'localhost' and split the two play steps into
    delegated and non-delegated versions.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>