Introduce the Lieutenant system into Neutron

Introduce the Lieutenant system into Neutron

As Neutron has grown, the need to continually evolve the way we land
code has changed. As the project has grown, it's become obvious we
need to subdivide responsibilites and merge rights across the
repository. The advanced services split [1] was one way in which we did
this, and it's proven effective. Plugin decomposition was another [2].
As a next step to evolve things, this changes the core reviewer
paradigm into a system of Lieutenants, all reporting to the PTL in the
project. The Lieutenants will be able to maintain their own core
review team. The idea of "super cores" will fade away. And we begin to
see the benefits of a layered, structured approach to managing
Neutron.

This proposal makes an attempt to comment on existing core reviewers.
While the idea of existing core reviewers being labeled as "supercores"
has been floated, the reality is a bit more nuanced than that, and I've
taken steps to address that with this patch. It should be noted that like
all things in Neutron governance, the system is based on a mutual trust.

[1] http://specs.openstack.org/openstack/neutron-specs/specs/kilo/services-split.html
[2] http://specs.openstack.org/openstack/neutron-specs/specs/kilo/core-vendor-decomposition.html

Change-Id: Ia4060066a300e851172c0fd7a2910ce53a6c649e