Move mechanisms to override stack docker images to profiles

[jsoriano]

There are currently two main ways to override docker images used by the compose provider of elastic-package stack:

• Use the image_ref_overrides setting in the .elastic-package/config.yml file.
• Use the *_IMAGE_REF_OVERRIDE environment variables.

Both mechanisms are undocumented, barely supported and tested, and are not local to profiles, while the rest of stack resources and settings are.

We should establish a single supported mechanism for this, that works more like a profile setting, as other settings we have been recently adding (see #1211). And we should document it.

[mrodm]

Just add a note that a new doc page was added to document how to use custom images:
#1323

Doc link: https://github.com/elastic/elastic-package/blob/60ee15405bfc1f759b745deffc56b7c2d33bdebe/docs/howto/custom_images.md

Even that, totally agree with establishing a single supported mechanism for this 👍