Add probes.allowed_groups option to restrict group assignments.
testsuite: fixed unit tests for Probes allowed_groups option
doc: defined when probes allowed_groups setting was added
Probes: added missing docstring
doc: Remove unkept bcfg2.conf page

The man pages are stored in our documentation now so this is obsolete.

Signed-off-by: Sol Jerome <sol.jerome@gmail.com>