Extract projects.yaml loader into a module.

* jeepyb/cmd/close_pull_requests.py, jeepyb/cmd/fetch_remotes.py,
jeepyb/cmd/manage_projects.py: Consume the new jeepyb.projects
module and stop loading the projects.yaml file directly.

* jeepyb/projects.py: New module, extracted from manage_projects.py
and refactored to load a somewhat simplified projects.yaml format.
This is a step toward further streamlining of the format.

Change-Id: I6a05536f016d1327844a652444f89bfde08ac6fa
Reviewed-on: https://review.openstack.org/20104
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins