Separate host group var in hosts plays

The old host group used in lxc_hosts and openstack_hosts was identical
in both plays. In some (if not many) environments, there is a
difference as some openstack_hosts do not have lxc containers.

In some situations such as diskless hypervisors, deploying the
lxc_hosts role to these nodes that do not use containers causes
unnecessary ramdisk clutter.

This minor change just separates the two variable references in these
plays so the operator can set for example:

lxc_host_group: "hosts:!compute_hosts"

Then my openstack_hosts play will still run on compute hosts as
expected.

Change-Id: Ib2b157b0e139f30792558535657d5d279c6a5fcc

playbooks/lxc-hosts-setup.yml

@@ -14,7 +14,7 @@
 # limitations under the License.
 
 - name: Basic lxc host setup
-  hosts: "{{ host_group|default('hosts') }}"
+  hosts: "{{ lxc_host_group|default('hosts') }}"
   max_fail_percentage: 20
   user: root
   roles:

playbooks/openstack-hosts-setup.yml

@@ -14,7 +14,7 @@
 # limitations under the License.
 
 - name: Basic host setup
-  hosts: "{{ host_group|default('hosts') }}"
+  hosts: "{{ openstack_host_group|default('hosts') }}"
   max_fail_percentage: 20
   user: root
   roles: