interface for loading dummy profile

[ltalirz]

So far, loading a dummy profile (no connection to a DB) has been necessary only for building the documentation on "readthedocs". The code necessary to do this therefore currently is a bit hackish and contains variables specific for this use case, see

/~https://github.com/aiidateam/aiida-core/pull/3374/files#diff-1ec61223727a10e5e4c74c68336809fcR261-R274

Loading a dummy profile could be useful not just on read the docs but in other contexts as well (e.g. for a quick test that doesn't need the DB... perhaps more).
But even if it is just used on RTD for the moment, it would be nice to provide an easier way for developers to load a dummy profile in their sphinx conf.py:

I propose to either

1. add a function load_dummy_profile() or
2. add an optional argument dummy_profile (or similar) toload_profile

In the first case, the function could live not at the top level but only under aiida.manage.configuration to allow for some time to converge behavior and user interface.

mentioning @sphuber @giovannipizzi for comment