Submit tasks to be executed

As a debusine user with an appropriate token, I can submit tasks to be executed with the debusine command line interface. I can pass arbitrary task data with some YAML input and I get back a task identifier that I can then later use to perform further queries on the task that I scheduled. The output is also YAML formatted to make it easy to script.

$ debusine schedule_task sbuild <<END
package: http://..../package_1.2-3.dsc
distribution: jessie
architecture: amd64
upload_method: dput
upload_target: freexian-elts
END
result: success
message: Task registered on debusine.freexian.com with task_id 123456.
task_id: 123456

If the given task is not known by the debusine server, we could get back a failure:

$ debusine schedule_task autopkgtest <<END
[...]
END
result: failure
message: Task autopkgtest is not known by debusine.freexian.com.