Improve the deployment elasticsearch keystore documentation

[marclop]

Description

The current ecctl help for the keystore commands isn't very good and leads to users of ecctl being confused as to what the exact format of the specified file should be.

$ ecctl deployment elasticsearch keystore set -h
Changes the contents of the Elasticsearch resource keystore from the
specified deployment by using the PATCH method. The payload is a partial payload where
any ignored current keystore items are not removed, unless the secrets are
set to "null": {"secrets": {"my-secret": null}}.

Usage:
  ecctl deployment elasticsearch keystore update <deployment id> [--ref-id <ref-id>] {--file=<filename>.json} [flags]

Aliases:
  update, set

Flags:
  -p, --file string     Required json formatted file path with the keystore secret contents.
  -h, --help            help for update
      --ref-id string   Optional ref_id to use for the Elasticsearch resource, auto-discovered if not specified.

Implementation

We should at least add a couple of examples to the documentation and define the expected file format.

References

• ESS API KeystoreContents model.
• Elasticsearch GCP credentials example.

[marclop]

Just noticed that the shorthand --file flag is incorrectly set to -p, I'll fix it in the same PR to be -f