BibSched: enforce CFG_BIBTASK_VALID_TASKS on CLI
BibSched: enforce CFG_BIBTASK_VALID_TASKS on CLI

* Enforce CFG_BIBTASK_VALID_TASKS check also when bibtasks are
  directly called via CLI. (closes #220)

inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio