Implement verdi config

[sphuber]

Fixes #2208

The new command verdi config replaces the various commands under
verdi devel that were used to list, get, set and unset configuration
options, that were used to be called "properties". Since the term
"property" is a reserved keyword, it was decided to rename them to
"options", which is also the term used by "git". The interface of
verdi config also mirrors that of git config. That is to say
to get the value of an option simply call git config <option_name>
and to set it git config <option_name> <option_value>. To unset
it, the --unset flag can be used. Finally, the getting, setting
or unsetting can be applied to a certain "scope", meaning to be
configuration wide or profile specific.

To make the implementation of this command simple and clean, the
bulk of the work was in refactoring the definition, construction and
operation on the configuration of an AiiDA instance. This is now
represented by the Config class, through which these configuration
options can be set or unset as well as retrieved.

[coveralls]

Coverage decreased (-6.9%) to 61.239% when pulling f46300b307c9f89a65bec6e2b5b6f95894fb3bbe on sphuber:fix_2208_verdi_config into 1e7e25e on aiidateam:provenance_redesign.

[coveralls]

Coverage increased (+0.05%) to 68.182% when pulling 83b35fb on sphuber:fix_2208_verdi_config into 1e7e25e on aiidateam:provenance_redesign.

[giovannipizzi]

Question: are the "mock-profile" part done in such a way that even if we run concurrently (e.g. in Jenkins) the tests both for Django and SQLAlchemy, they don't step on each other toes?

[sphuber]

The mock profile function currently just generates a dummy profile, with dummy variables. The profile is not "actually" used. The only shared part might be the repository path, but since the aiida.backends.tests.utils.configuration.with_temporary_config_instance decorator creates a completely temporary directory in which everything is created, these should never overlap.

Note that currently the creation of a new "profile" is still done manually by the create config functions that are called by quicksetup and setup, i.e. they manually build a dictionary with the required keys. In a future PR this should be refactored, as the Profile class is the only one that should now about these keys.

[giovannipizzi]

Just a couple of comments (one above, one to know if this PR changes the way the AIIDA_PATH is used). apart from this, good to go!!

[giovannipizzi]

Are the new command line verdi commands described? (couldn't see the change)

[sphuber]

I have added the new command to the verdi docs

[sphuber]

And to answer your other question, no this does not change the functioning of AIIDA_PATH, it has just been moved to aiida.manage.configuration.settings.