* SQLStore: customise the limit of retrieved datasources per organisation

* update all suggestions regarding nil or 0 as default

* Apply suggestions from code review

Co-authored-by: Emil Tullstedt <emil.tullstedt@grafana.com>

* correct default.ini description + adding unittest

* Apply suggestions from code review

Co-authored-by: Sofia Papagiannaki <papagian@users.noreply.github.com>

* modify unittest name

Co-authored-by: Emil Tullstedt <emil.tullstedt@grafana.com>
Co-authored-by: Sofia Papagiannaki <papagian@users.noreply.github.com>