bgpd: Implement group-overrides for peer attrs

bgpd: Implement group-overrides for peer attrs

This commit introduces BGP peer-group overrides for the last set of
peer-level attrs which did not offer that feature yet. The following
attributes have been implemented: description, local-as, password and
update-source.

Each attribute, with the exception of description because it does not
offer any inheritance between peer-groups and peers, is now also setting
a peer-flag instead of just modifying the internal data structures. This
made it possible to also re-use the same implementation for attribute
overrides as already done for peer flags, AF flags and AF attrs.

The `no neighbor <neigh> description` command has been slightly changed
to support negation for no parameters, one parameter or * parameters
(LINE...). This was needed for the test suite to pass and is a small
change without any bigger impact on the CLI.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>