Makefile: Fix unconditional metad files in ENABLE_METADATA

Since ENABLE_METADATA is optional, they should have their own
optional section, just like ENABLE_MOND does.
NEWS: Mention job filters and ad-hoc filtering

They will be part of Ganeti 2.13.
Build confd by default

The only reason why it was optional was its dependency on regex-pcre,
which was an optional Ganeti dependency.

Job filter rules require rexex-pcre unconditionally, so now confd
requires no additional depdendencies and can be built by default.