webhook_url is not mandatory in garm v0.1.4 but needed by updating to v0.1.5

[bavarianbidi]

in garm v0.1.4 it's not mandatory to have webhook_url set.

By updating garm from v0.1.4 to v0.1.5, the /controller-info endpoint will fail as webhook_url got not converted into the database.

garm-cli-v0.1.5 controller show
Error: [GET /controller-info][409] controllerInfoConflict {"error":"urls_required","details":"Missing required URLs. Make sure you update the metadata, callback and webhook URLs"}

[gabriel-samfira]

Right. In it's current form (no scale sets yet), the webhook_url is kind of mandatory. We should probably relax this, as it won't really be mandatory once we have scale sets. It will only be mandatory for the current pools. So we can probably set this to optional.