Add read/write and grant config info for external etcd in cluster guide

[hillaryfraley]

Description

Add information about configuring read/write access and grant permission for etcd auth.

Motivation and Context

#2184

Review Instructions

This feels like it might be a little thin. Am I missing some details in my explanation?

Also, I want to make sure I added this in the best place in the docs.

[cwjohnston]

Sorry this has lingered so long. I spent some time testing this myself and found a few things I think we should add.

[cwjohnston]

Suggested change

	/opt/etcd/etcdctl role grant-permission sensu_readwrite readwrite --from-key '/sensu.io/'
	# Add the sensu user, you'll be prompted for the password interactively
	etcdctl user add sensu
	# Create a role
	etcdctl role add sensu_readwrite
	# Give that role permission to read and write under the /sensu.io/ keyspace
	etcdctl role grant-permission sensu_readwrite readwrite --from-key '/sensu.io/'
	# Grant the sensu user the role
	etcdctl user grant-role sensu sensu_readwrite

I found additional steps necessary here.

[tarcinil]

This will need .initialized still too. I just built a new cluster and it failed to init without having access to that key.

[hillaryfraley]

@tarcinil do you mean we need to incorporate http://localhost:1313/sensu-go/5.20/installation/install-sensu/#3-initialize by reference? Or do you mean something else?

[hillaryfraley]

Closed in favor of #2792