Add task.count to params to post_managed_build_start

This commit removes @count as an instance variable of the BuildManager class,
since its only ever used as a parameter to the managed build start method.
Instead we just pass it in as part of the call to the method. This will rely on
"count" living somewhere else - we're going to add it to the task itself, by
monkey-patching @task_count as a Rake::Task instance variable.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Monkey patch in @count into Rake::Task

This allows us to keep track of how many different tasks we anticipate a task
will kick off. This will allow us to know, for example, how many internal
"pl:deb" calls a pl:jenkins:deb_all call will make, which will then allow us to
track progress of the individual tasks from the build manager. This commit
includes a basic test to ensure instances of Rake::Task have accessors for
@count.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>