[sources] Check sources configuration is fine on creation/modification
Reintroduce usage of 'source.check_config' which had almost disappeared, as
well as tests for the currently detected errors.
Part of the system source specific checking done in syncsources reimplemented in
a specific check_config implementation.
Tests are dispatched among ldap / datafeed and syncsources tests but are not
strictly correctly located (notably syncsources tests behaviour of the native
source's check_config).
The system source url checking part which disappears from syncsources will be
reintroduced in a follow-up.