Load cell data from a configuration file

**WIP**

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.

TODO:

* Update the test suite to test the newly-added code and verify
  behavior when cells_config is provided

Change-Id: I7046ce55a0a294293c1b1a5fb0f092aeb891ee01