builds: move create to api endpoint
This includes removing some extra code that's not needed anymore and adding a common BuildClient class to handle the builds api requests.
Note that it removes some parameters that are not required anymore (as they are on the API side) or that we might not want to expose to users as-is (image-name and image-tag).
Bug: #337218
Signed-off-by: David Caro dcaro@wikimedia.org