Make it possible to reattach NEGs automatically

[fdfzcq]

The current behaviour of Autoneg introduces race conditions for us, the scenario goes:

• A service is deployed with existing BackendService and NEGs
• Autoneg connects the service to existing NEGs and updates its annotations
• User removes BackendService resource
• Autoneg does nothing because the NEGs status stays unchanged but the service is actually detached from the NEGs
• User recreates a new BackendService
• Autoneg will do nothing because the NEGs status stays unchanged

The proposed solution here without us doing big architectural change is to make Autoneg reconcile periodically. Since this is a big behaviour change, I added a flag to enable it but defaults to false. Along with another flag to configure the reconciliation period.

[rosmo]

Don't get me wrong, I support this, but couldn't the user just delete the status annotation to re-reconcile?

[fdfzcq]

@rosmo Yes, that was a workaround, but it adds toils. And we think this kind of mishaps can happen without users notice and cause incident.

[fdfzcq]

@rosmo Hello, could you re-review the PR? :D

[rosmo]

LGTM

[joshw123]

Hello, Do you know when this will be released? :) Thank You!

[rosmo]

@joshw123 I'll roll out a new pre-release after issue in #113 is solved.