Add option to disable kube-router BGP graceful restarts

[rkusler-intermedia]

What would you like to be added:
The option to set --bgp-graceful-restart to false when starting kube-router. The default should be to leave it true, but we'd like the ability to disable it.

Why is this needed:
Currently the --bgp-graceful-restart option is hard coded in roles/network_plugin/kube-router/templates/kube-router.yml.j2. This can cause problems in certain situations where the far-end router isn't configured to use graceful restarts. When we run kubespray and it restarts the kube-router container, kube-router stops advertising all of its LoadBalancer Service IPs resulting in a prolonged outage. Our workaround is to modify the kube-router.yml.j2 manifest to disable the feature, but it would be better if it were simply a flag so that we don't have to maintain a kubespray fork.

Related issues:
cloudnativelabs/kube-router#1323
cloudnativelabs/kube-router#1486
osrg/gobgp#2596