Refine command override create
* Pass username to BodhiClient class
* Use datetime.utcnow instead of datetime.now, which is same as
what bodhi-client API does
* Print created override by call to BodhiClient.override_str rather than
printing by fedpkg itself
* Show fedpkg or fedpkg-stage in description accordingly
* Update tests.
Signed-off-by: Chenxiong Qi <cqi@redhat.com>