Load cell data from a configuration file

Cells currently keeps all inter-cell communication data, including
usernames and passwords, in the database.  This is undesirable and
unnecessary, since cells data isn't updated very frequently.  This
change allows cells data to be drawn from a JSON file specified via
a new [cells]cells_config option.  When specified, the database is
no longer consulted when reloading cells data.

Implements blueprint eliminate-clear-passwords-from-cells-table.

DocImpact: Cells may now optionally be configured through a JSON-
formatted file.  The file will need the columns present in the Cell
model (excluding common database fields and the 'id' column).  The
queue connection information must be specified through a
'transport_url' field, instead of 'username', 'password', etc.  The
transport_url has the following form:

    rabbit://<username>:<password>@<hostname>:<port>/<virtual_host>

The scheme may be either 'rabbit' (shown above) or 'qpid'.

Change-Id: I7046ce55a0a294293c1b1a5fb0f092aeb891ee01