Var target security playbook host group

Set the playbook host scope to a var named "security_host_group" to
allow operators to filter the hosts targeted by the play.

Change-Id: I7095eb8658337d165068c5d8034532211c4bd273

playbooks/security-hardening.yml

@@ -18,7 +18,7 @@
 # hardening standards to a system.
 
 - name: Apply security hardening configurations
-  hosts: "{{ host_group|default('hosts') }}"
+  hosts: "{{ security_host_group|default('hosts') }}"
   gather_facts: "{{ gather_facts | default(True) }}"
   user: root
   roles:

releasenotes/notes/security-playbook-hosts-var-a9b71f3dbcda2cad.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - The security-hardening playbook hosts target can now be filtered using the
+    ``security_host_group`` var.