We already have many different jobs defined for Zuul in Neutron repo.
Up to now all those jobs were in .zuul.yaml file but this file growth
really big and it was hard to read and navigate in it.
So this patch proposes to move jobs definitions to zuul.d directory to
separate yaml files per "job types". So there will be now separate files
with definitions for:
* grenade jobs,
* base jobs like functional and fullstack,
* rally jobs,
* tempest multi node jobs,
* tempest single node jobs,
* tripleo jobs.
And maybe others in the future.
Change-Id: Ia77f59fded0e6e0ae328d866a7486af02b50c2d7