Apply the variable naming schema

[afeefghannam89]

Problem

The naming schema agreed in #542 is not applied yet. Several variables either do not say what
they hold, or use a different word/form than the rest of the collection for the same concept.

Want

Apply the schema from #542 to the variables listed below.

current	new	why
elasticstack_ca_pass	elasticstack_ca_passphrase	protects the CA key, so it is a passphrase; _pass is ambiguous (rules 3)
elasticstack_initial_passwords	elasticstack_initial_passwords_file	the value is a file path (rule 7)
elasticsearch_bootstrap_pw	elasticsearch_bootstrap_password	pw is used nowhere else (rule 3)
logstash_role_indicies_names	logstash_role_indices_names	typo (rule 8)
logstash_role_indicies_privileges	logstash_role_indices_privileges	typo (rule 8)
module option indicies of elasticsearch_role	indices	typo (rule 8); the module already translates it to indices= internally
elasticsearch_ml_enabled	elasticsearch_ml_enable	one boolean suffix (rule 5)
elasticsearch_monitoring_enabled	elasticsearch_monitoring_enable	one boolean suffix (rule 5)

Renaming the module option is breaking for anyone calling
netways.elasticstack.elasticsearch_role directly, not only for role users.

Two variables that need a decision, not a rename

• elasticsearch_disable_systemcallfilterchecks - negative polarity (rule 4), but it only ever
  applies on Elastic Stack 7 (templates/elasticsearch.yml.j2). It should be removed with
  the Elastic Stack 7 support ([Feature]: Remove the support of elasticstack 7 #446) instead of renamed.
• logstash_no_pipelines - negative polarity (rule 4), but it also overlaps
  logstash_manage_pipelines; tasks/main.yml checks both together. Merging the two into one
  variable is a design decision, not a mechanical rename.

Out of scope - owned by other issues

• elasticstack_enable_repos, elasticstack_repo_url, elasticstack_repo_key,
  elasticstack_rpm_workaround, elasticstack_kibana_port, elasticstack_kibana_host,
  elasticstack_temperature -> renamed while being moved in [Feature]: Move single-role variables out of the elasticstack meta role #485
• elasticstack_password, elaticsearch_cluster_set_up and the other internal
  register / set_fact variables -> Adopt an internal-variable naming convention (_ / __) and keep internals out of argument_specs #495

Scope - one PR per role

• elasticsearch
• kibana
• logstash
• beats
• repos
• elasticstack
• elasticsearch_role module option

Depends on #542. Should land after #495 and #485 so that no variable is renamed twice.