Pin etcd to latest 3.x version

[arm4b]

Currently we rely on etcd 2.2.5, pinned by default in external etcd Helm chart dependency:
https://github.com/helm/charts/blob/b7f4e8e17aa9cd2adba42756bf7d56f36d90449b/incubator/etcd/values.yaml#L10-L12

Per discussion with @Kami, etcd 3.x is needed for group membership functionality, required by st2.

[arm4b]

Closing as #94 etcd operator is not the way and should be replaced with redis for [coordination] backend.