Add placement as required project to functional py36 and 37

The functional tests are configured to install placement from git
so that they test with placement-from-master. In the py27 and py35
functional tests, placement is a part of required-projects, which
means that depends-on to a particular patch of placement can work
via tox-siblings[1].

This patch adds the same functionality for py36 and py37 functional
tests, which currently run only in the experimental queue.

[1] https://docs.openstack.org/infra/manual/zuulv3.html#installation-of-sibling-requirements

Change-Id: Ia40aba6d3036fea2bc62fdf76e8e221b552f1cdc

.zuul.yaml

@@ -83,6 +83,8 @@
       Run tox-based functional tests for the OpenStack Nova project
       under cPython version 3.6 with Nova specific irrelevant-files list.
       Uses tox with the ``functional-py36`` environment.
+    required-projects:
+      - openstack/placement
     irrelevant-files: *functional-irrelevant-files
     vars:
       tox_envlist: functional-py36
@@ -97,6 +99,8 @@
       Run tox-based functional tests for the OpenStack Nova project
       under cPython version 3.7 with Nova specific irrelevant-files list.
       Uses tox with the ``functional-py37`` environment.
+    required-projects:
+      - openstack/placement
     irrelevant-files: *functional-irrelevant-files
     vars:
       tox_envlist: functional-py37