Fix django-admin commands

Some django-admin commands used to inherit from django’s NoArgsCommand.
Therefore their implementation was placed in handle_noargs() method.

A previous commit has changed the django-admin commands all over synnefo to
inherit from SynnefoCommand (which inherits from django’s BaseCommand),
without changing the implementation method to handle().
As a result, these commands fail with NotImplementedError.

This commit fixes this issue.

Closes #103
pithos: Restore SQLite backend module.

The Pithos+ backend can work with several database backend modules.
Currently there are modules for SQLAlchemy and SQLite.
However, the SQLite module was broken by the changes introduced by the support
of snapshots.
This commit restores the SQLite module.

Closes #102
astakos: Create system project on user acceptance

Delay the creation of user's system project until the user is accepted,
to avoid polluting the database with unneeded project objects.

When creating the system project, extra checks are needed for possible
UUID collisions. If an existing project is found with the same UUID as
the user-to-be-accepted, the user's UUID is changed on the fly. However,
the probability of a UUID collision is close to zero.
astakos: Change the display name of system projects

Change the name of projects from "base:UUID" to "system:UUID", so as
to avoid confusion, since the name is visible in management commands.