Update contributor/placement.rst to contemporary reality
Update contributor/placement.rst to contemporary reality

This udpates the placement contributor documentation to reflect the
current state of affairs. Ininitially this was just going to be an
update to reflect that the oslo versioned objects had moved. However,
while reviewing the rest of the doc it became clear there were other
issues that should be resolved. Doing so in one shot helps to maintain
the continuity of the document and doing it piecemeal means it is less
likely to happen.

Change-Id: Ia8d4861799e27a30108dd465d15828c5a1943819

mikalstill/nova,openstack/nova,klmitch/nova,klmitch/nova,klmitch/nova,phenoxim/nova,openstack/nova,mikalstill/nova,rahulunair/nova,gooddata/openstack-nova,rahulunair/nova,mahak/nova,phenoxim/nova,gooddata/openstack-nova,gooddata/openstack-nova,mahak/nova,mikalstill/nova,openstack/nova,klmitch/nova,mahak/nova,gooddata/openstack-nova,rahulunair/nova