Document etcd-cipher-suites backend config option

[apaskulin]

Description

• Updates the backend reference to include the etc-cipher-suites attribute
• Updates the example backend config file
• Updates the securing Sensu guide
• Added defaults based on https://github.com/etcd-io/etcd/blob/master/pkg/tlsutil/cipher_suites.go

To do:

• Add more information in the reference and guide (Reference: Secure socket communication protocols and ciphers should be configurable sensu-go#2914)
• Add realistic example
• Open PR for example file in sensu-enterprise-go

Motivation and Context

Closes #1468

[echlebek]

Should we list the supported ciphers? They can be found here:

https://golang.org/pkg/crypto/tls/#pkg-constants

The ciphers from TLS 1.0 - 1.2 are available, TLS 1.3 ciphers are not available. (But TLS 1.3 can be enabled in another way)

[apaskulin]

Is there a default value?

[echlebek]

No - an empty value will lead to the default etcd cipher suites being used. (I'm not sure what they are.)

[apaskulin]

Based on this doc: "empty will be auto-populated by Go". So maybe the values listed here would be the defaults?

[echlebek]

That appears to be the case.

https://github.com/etcd-io/etcd/blob/master/embed/config.go#L187

[cwjohnston]

I think this looks good for the release. It would be ideal if we can identify and document the defaults, but we don't need to be blocked by that.