[Umbrella]: Collection-wide variable naming and specification hygiene

[afeefghannam89]

Problem

Variable naming and the role argument specifications are handled ad hoc. Names follow no
documented schema, internal runtime variables are indistinguishable from user-facing ones and
are partly declared in defaults/main.yml and meta/argument_specs.yml, and single-role
variables sit in the elasticstack meta role. As a result every issue that renames something
re-decides the same rules.

Want

One documented naming schema, applied across all roles, with internal variables kept out of the
user-facing contract.

Children

• Define the collection-wide variable naming schema #542 - defines the rules the others use to pick target names
• Adopt an internal-variable naming convention (_ / __) and keep internals out of argument_specs #495 - blocked by Define the collection-wide variable naming schema #542
• Check the argument specification in every role #474 - blocked by Adopt an internal-variable naming convention (_ / __) and keep internals out of argument_specs #495
• [Feature]: Move single-role variables out of the elasticstack meta role #485 - blocked by Define the collection-wide variable naming schema #542
• Apply the variable naming schema #544 - blocked by Adopt an internal-variable naming convention (_ / __) and keep internals out of argument_specs #495 and [Feature]: Move single-role variables out of the elasticstack meta role #485

#474 has to run after #495, because #495 removes internal variables from defaults/main.yml and
meta/argument_specs.yml - auditing first would cover entries that #495 then deletes. #544
comes last so that no variable is renamed twice.

elasticstack_password needs both an internal prefix and a meaningful name. It is touched only
once, in #495, with the target name __elasticstack_elastic_password.