Kubernetes project is about to set etcdv3 as default storage engine in
1.6. This patch allows to specify particular backend for
kube-apiserver. User may force the option to etcdv3 for new environment.
At the same time if the environment uses v2 it will continue uses it
until user decides to upgrade to v3.

Signed-off-by: Sergii Golovatiuk <sgolovatiuk@mirantis.com>