Rewrite method to create bodhi update
    
    Executing command bodhi may have some problems. One major issue is it
    does not return non-zero when fail to create update if something wrong
    with the parameters within template.
    
    Calling API directly will bring us full control of the creation workflow
    to handle potential failures without relying on the behavior of command
    bodhi.
    
    In addition, this rewrite also validates some parameters before
    creation.
    
    Resolves: rhbz#1492480
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>