add multiple DB backend-support - etcd

[naorlivne]

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Expected/Wanted Behavior

Similar to #4 etcd is another good match for Nebula requirements from the backend DB - it should be another option for multiple DB supported as one of the backends used by etcd, allowing the api-managers to watch for values changed and pull the changes for a local memory only cache on them will greatly improve the API speed while allowing users to chose to use whatever they prefer to be the backend DB.

Actual Behavior

Mongo is the only possible backend DB