[multi-node-bridge] Allow to skip openvswitch installation

In neutron-multinode jobs where we use ovs/ovn from source,
we want to use multi-node-bridge role only for bridge
configuration but not for ovs installation. In the job
we install ovn and openvswitch before calling this role to
configure the bridges.

Adding a role var 'install_ovs' to allow skipping ovs
installation and service start, it's default to true so
no change in current behavior of the role.

It's an alternative approach to [1].

[1] https://review.opendev.org/c/zuul/zuul-jobs/+/762650

Related-Bug: #1904117
Change-Id: I64942679520681bdf7f953c0a3c7fc0d13e77856

roles/multi-node-bridge/README.rst

@@ -74,3 +74,9 @@ inventory in order to work:
    :default: 23
 
    The IP address range CIDR/subnet.
+
+.. zuul:rolevar:: install_ovs
+   :default: true
+
+   Whether or not to install openvswitch. It can be set to false
+   when ovs installation is taken care outside of the role.

roles/multi-node-bridge/defaults/main.yaml

@@ -6,3 +6,4 @@ bridge_configure_address: true
 bridge_address_prefix: 172.24.4
 bridge_address_offset: 1
 bridge_address_subnet: 23
+install_ovs: true

roles/multi-node-bridge/tasks/common.yaml

@@ -49,6 +49,7 @@
     name: "{{ ovs_package }}"
     state: present
   when:
+    - install_ovs|bool
     - ansible_distribution != 'Gentoo'
 
 - name: Install openvswitch (Gentoo)
@@ -58,6 +59,7 @@
     state: present
     jobs: 8
   when:
+    - install_ovs|bool
     - ansible_distribution == 'Gentoo'
 
 - name: Ensure openvswitch is started
@@ -66,6 +68,8 @@
     name: "{{ ovs_service }}"
     state: started
     enabled: yes
+  when:
+    - install_ovs|bool
 
 - name: Remove RDO repository files
   become: yes