Add option to specify backing store etcd

[gadkins]

This issue implements the interface for UffizziCloud/uffizzi-cluster-operator#57

Summary

Add the option to specify the uCluster backing store, either etcd or sqlite.

Example usage

uffizzi cluster create --backing-store=etcd

Possible values for should be etcd or sqlite.

Help page

uffizzi cluster create -h
uffizzi-cluster-create - create a cluster
================================================================

## SYNOPSIS
    uffizzi cluster create [CLUSTER_NAME]

## DESCRIPTION
    Creates a new cluster. If no CLUSTER_NAME is specified,
    the cluster is created with the auto generated name.

    For more information on Uffizzi clusters, see:
    https://docs.uffizzi.com/references/cli/

## POSITIONAL ARGUMENTS
    CLUSTER_NAME
        Name of the cluster to create  

## FLAGS
    --backing-store="<etcd-or-sqlite>"
        Set the cluster backing store.
        Use value "etcd" to optimize for IOPS.
        Use value "sqlite" to optimize for cluster start-up time.

    --kubeconfig="/path/to/your/kubeconfig"
        Path to kubeconfig file

    --manifest="/path/to/your/manifest"
        Path to manifest file

    --name
        Option is deprecated and will be removed in the newer versions.
        Please use a positional argument instead: uffizzi cluster create my-awesome-name.

    --update-current-context
        Update current-context in kubeconfig file
        Default is true

    --output=pretty-json
    --output=json
        Use this option for formatting output.

## EXAMPLES
    To create a cluster with the auto generated name, run:

        $ uffizzi cluster create

    To create a cluster with name, run:

        $ uffizzi cluster create my-cluster

    To create a cluster from a manifests directory, run:

        $ uffizzi cluster create my-cluster --manifest=manifests/

    To create a cluster optimized for IOPS, set the backing store to etcd:
        $ uffizzi cluster create --backing-store=etcd