Add placement_install_method for distro installations

This patch adds placement_install_method of distro in the extra
vars file that we use for testing in order for tests to properly
run for Placement.

Change-Id: I878e8c479d8963b339b5e1f6031f649c0f87082f

test-distro_install-vars.yml

@@ -53,6 +53,7 @@ neutron_install_method: distro
 nova_install_method: distro
 octavia_install_method: distro
 panko_install_method: distro
+placement_install_method: distro
 rally_install_method: distro
 sahara_install_method: distro
 searchlight_install_method: distro