tahoe add-grid-manager

[meejah]

A CLI method of adding a new grid-manager would be nice. This would add a single key to the [grid_managers] section of the config.

[meejah]

Pulled out of the grid-manager doc/proposal:

tahoe add-grid-manager (PROPOSED)
`````````````````````````````````

This takes two arguments: ``name`` and ``public-identity``.

The ``name`` argument is a nickname to call this Grid Manager. A
client may have any number of grid-managers, so each one has a name. A
client with zero Grid Managers will accept any announcement from an
Introducer.

The ``public-identity`` argument is the encoded public key of the Grid
Manager (that is, the output of ``grid-manager                                                                         
public-identity``). The client will have to be re-started once this
change is made.