mgr/cephadm: safe status/health access in node-proxy agent and inventory

mgr/cephadm: safe status/health access in node-proxy agent and inventory

This adds helpers in NodeProxyEndpoint and NodeProxyCache to safely
read status.health and status.state.

In NodeProxyEndpoint, methods _get_health_value() and _get_state_value()
are used in get_nok_members() to avoid KeyError on malformed data.

In NodeProxyCache, _get_health_value(), _has_health_value(),
_is_error_status(), and _is_unknown_status() are used in fullreport()
and when filtering 'non ok' members instead of accessing
status['status']['health'] inline.

Fixes: https://tracker.ceph.com/issues/74749
Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>