Upgrade ceilometer first

Ceilometer installs code into the paste pipeline of other projects,
which means that if other projects are upgraded first, their upgraded
versions may get started with old ceilometer code. As we are finally
starting to pin requirements on stable, this prevents things like
Swift from starting.

This seems like an architectural flaw in Ceilometer, as it should be
consuming externalized interfaces not sticking itself into the
pipeline.

Alternative fix is to remove ceilometer support from grenade entirely,
as the project doesn't provide the kind of upgrade model that we
expect in OpenStack. However, for now, a reordering should get things
back working and let people merge code.

There also remains an oslo.db coupling issue, which we need to get
around by artificially installing an old oslo.db before we get
started.

Change-Id: I40e151cf92cad95ddea9919537970ab471d4511a
Related-Bug: #1403024