Adapt watcher for ENABLE_CONFD

If confd is disabled, do not automatically restart it. Furthermore, we
can't run maintenance actions if it is disabled so log a warning.

Note that I haven't completely disabled the NodeMaintenance class with
ENABLE_CONFD = False because I think they are at two different levels
(e.g. we might have other maintenance actions done even with confd
disabled).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>