Make peadm::uninstall private, multi-target

There isn't a reason to restrict this to operating on only one target at
a time. Additionally, we should use the convention $targets, so that the
CLI flag --targets or -t will auto-populate this parameter.

Because this may not universally work on all systems, and we don't
handle the situation where Puppet might be installed, but the
puppet-enterprise-uninstaller script isn't prresent, mark this plan
experimental/private.

plans/uninstall.pp

@@ -1,13 +1,15 @@
+# This plan is in development and currently considered experimental.
+#
+# @api private
+#
 # @summary Single-entry-point plan for uninstalling Puppet Enterprise
-
 plan peadm::uninstall (
-  Peadm::SingleTargetSpec  $primary_host,
+  TargetSpec $targets,
 ) {
   peadm::assert_supported_bolt_version()
 
-  $primary_target = peadm::get_targets($primary_host, 1)
-  $uninstall_result = run_task('peadm::pe_uninstall', $primary_target)
+  $uninstall_results = run_task('peadm::pe_uninstall', $targets)
 
-  return([$uninstall_result])
+  return($uninstall_results)
 }