Add support for modifying default contact configuration

[Sandor Zeestraten]

It should be possible to modify the default contact definition options:

* service_notification_period
* host_notification_period
* service_notification_options
* host_notification_options
* service_notification_commands
* host_notification_commands

This together with #1864969 blocks the ability to enable notifications for schedule downtimes which requires the option `host_notification_options` and `service_notification_options` to include `s`.

See https://assets.nagios.com/downloads/nagioscore/docs/nagioscore/3/en/objectdefinitions.html#contact for valid contact definitions.

[Alvaro Uria (aluria)]

Released at cs:~llama-charmers-next/nagios until next stable/20.05 (which will be in cs:nagios).