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>