Add placement as required project to functional py36 and 37
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

mahak/nova,klmitch/nova,klmitch/nova,mahak/nova,rahulunair/nova,klmitch/nova,rahulunair/nova,rahulunair/nova,klmitch/nova,mahak/nova,openstack/nova,openstack/nova,openstack/nova