feat: Support to choose openstack backend plugins

[powerds]

Is this a bug report or feature request? (choose one): feature request

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openstack's modules are implemented to select plugin backend system or agents. for example, nova can select hypervisors like kvm, vmware, xen and so on, neutron can select agents like linuxbridge-agent, opencswitch-agent and so on.
So it needs to design to choose plugins in helm charts. in neutron-helm package case, it needs to configure to disable openvswitch-agent to do not deploy openvswitch-agent daemonset, and enable another agent like linuxbridge-agent daemonset and config map.
almost all openstack's modules have same concept to use plugins, so we need to design all charts can use.

[v1k0d3n]

@powerds thanks for checking out openstack-helm! so this feature is in fact the direction of openstack-helm, and we are working on this currently. we will have a couple backend options for cinder (as an example) to show developers exactly how we plan on addressing this feature request going forward (and before we enter openstack proper).

[powerds]

@v1k0d3n thanks. but it looks there's some difference between cinder multi backend function (#300) and neutron agent plugin function on openstack-helm. because in cinder case, there's no change of k8s resources. the cinder configuration acts in cinder-volume deployment so it's enough to fix cinder.conf. but in neutron case, ovs-agent, ovs-db, ovs-vswitchd daemonsets should be not deployed when we set up to use linuxbridge-agent. it means neutron-helm chart should control what *-agent daemonsets will be deployed or not. do we need new issue for neutron?