notify: add built-in config and 'origin' parameter

notify: add built-in config and 'origin' parameter

This allows us to define a (modifiable) builtin-config, which is
at the moment hardcoded in PVEContext

The 'origin' parameter indicates whether a config entry was created by
a user, builtin or a modified builtin.

These changes require context to be set for tests, so we set
PVEContext by default if in a test context. There might be a nicer
solution for that, but for now this should work.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>