[Bug]: Remove forced enablerepo from install tasks (breaks with externally-managed repos)

Follow-up to #312. All component install tasks (beats, logstash, kibana, elasticsearch — including elasticsearch-rolling-upgrade.yml) hardcode enablerepo: elastic-{{ release }}.x on RedHat. This breaks with externally-managed repos (Red Hat Satellite, mirrors): the repo has a different name and is enabled server-side, so the client shouldn't enable/disable repos.

Today each role carries two near-identical RPM install tasks that differ ONLY by this enablerepo — "rpm - full stack" (with it) vs "rpm - standalone" (without) — gated on elasticstack_full_stack. Remove the forced enablerepo entirely and merge the two into one install task, so installation no longer depends on elasticstack_full_stack. Same in the rolling-upgrade tasks.

This removes meaning (A) from the elasticstack_full_stack decomposition (#531).

Note: the collection does NOT update packages automatically — all installs use state: present; a package only changes when the user bumps elasticstack_version. Preventing elastic packages from updating during an OS-level yum update is a system concern (e.g. yum-plugin-versionlock), outside the collection's scope.