Convert cells to use a transport URL

Cells currently keeps all inter-cell communication data, including
usernames and passwords, in individual columns in the "cells" table
of the database.  This renders it difficult to specify alternate
transports (e.g., 0mq).  Work is currently in progress to add the
concept of "transport URLs," but the end goal is to remove this data
from the database entirely and store it in a configuration file.

This change adds transport URLs to cells in a very limited fashion;
only rabbit and qpid URLs are recognized, and no auxiliary data is
recognized.  This cleans up the database table, and will make it
easier to move cell data out of the database and into the
configuration file.

Partially implements blueprint eliminate-clear-passwords-from-cells-table.

DocImpact: None.  Transport URLs are not exposed to deployers in
           this patch.

Change-Id: I6b18f7643ab694f5ff34206b80865c40b1ec2680
Merge "Convert cells to use a transport URL"