Send websocket notify when general data updates

[Huw Wilkins]

When data for the 'general' endpoint changes (https://github.com/maas/maas/blob/master/src/maasserver/websockets/handlers/general.py) the websocket does not send notify messages.

[Jerzy Husakowski (jhusakowski)]

The data exposed on the 'general' endpoint may need to be split into separate endpoints, as the idea of the 'general' endpoint was to expose data that does not change.

[Jerzy Husakowski (jhusakowski)]

Would it be possible to make an inventory of things that UI needs to be notified on? There are more than 20 items in the 'general' endpoint and they don't all look like something that UI needs to be notified about.

[Huw Wilkins (huwshimi)]

These are the notifications that I think we need (at least that I can find at the moment):

osinfo
- when changing the default os in config the general osinfo needs to be updated with the new default: https://github.com/canonical/maas-ui/issues/148#issuecomment-545908773

default_min_hwe_kernel
- when changing the default min HWE kernel in config the general default_min_hwe_kernel needs to be updated with the new default

hwe_kernels
osinfo
default_min_hwe_kernel
architectures
- after images are synced these values need to be updated: https://bugs.launchpad.net/maas/+bug/1964143