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 AMQP 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.

WIP; still to do:

x Implement database migration
* Write appropriate migration test
* Add tests on the temporary parse_transport_url() function
* Update other tests

Change-Id: I6b18f7643ab694f5ff34206b80865c40b1ec2680