pithos: Fix tests

Fix tests challenging partial content update that used to fail with
ValueError: empty range for randrange().
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.