Cell Support

Yagi-workers are now created per cell. One rabbit per cell.
Also renamed the common role to stv3-common to prevent conflicts
with other playbooks.

Take note of ansible/roles/stv3-workers/defaults/main.yaml to
see an example of the expected format of the calling facts file.
Each entry in this list-of-dictionaries represents a rabbit server
and Swift container for that cell.

Also, multiple pipeline_workers are created based on the value
of num_pipeline_workers (defaults to 2). This is so a machine
can better utilize multi-core architectures.

Change-Id: I09c4d364878493546d6e23e019f01f5f4f38c37c