Complex (anti-)affinity policies

To meet typical telecommunication service provider SLAs the failure
of any given host cannot take down more than k VMs in each N+k pool
for a given application. More precisely, given that the pools are
dynamically scaled, it is a requirement that at no time is there
more than a certain proportion of any pool instantiated on the same
host.

An N+k pool is a pool of identical, stateless servers, any of which
can handle requests for any user. N is the number required purely
for capacity; k is the additional number required for redundancy.
k is typically greater than 1 to allow for multiple failures.
During normal operation N+k servers should be running. Conceptually
in this context a pool is roughly analogous to Nova's concept of a
"server group" though the latter does not support the type of policy
described in this proposal.

Affinity/anti-affinity can be expressed pair-wise between VMs, which
is sufficient for a 1:1 active/passive architecture, but an N+k pool
needs something more subtle. Specifying that all members of the
pool should live on distinct hosts is clearly wasteful. Instead,
availability modelling shows that the overall availability of an N+k
pool is determined by the time to detect and spin up new instances,
the time between failures, and the proportion of the overall pool
that fails simultaneously. The OpenStack scheduler needs to provide
some way to control the last of these by limiting the proportion of
a group of related VMs that are scheduled on the same host.