implements: blueprint weigh-host-by-hypervisor-version
Change-Id: I36b16a388383c26bdf432030bc9e28b2fd75d120

Current versions of mypy run with no-implicit-optional
by default.  This change gets Nova's mypy test environment
to pass again.

Change-Id: Ie50c8d364ad9c339355cc138b560ec4df14fe307

Like we did for conductor, this makes the scheduler lazy-load the
placement client instead of only doing it during __init__. This avoids
a startup crash if keystone or placement are not available, but
retains startup failures for other problems and errors likely to be
a result of misconfigurations.

Closes-Bug: #2012530
Change-Id: I42ed876b84d80536e83d9ae01696b0a64299c9f7

Ran into this randomly today, if a test sets
CONF.scheduler.enabled_filters to a non-default value, the affinity
support global variables will be set to False which can affect
subsequent test runs that expect the default configuration (affinity
filter support enabled).

Example error:

  WARNING [nova.scheduler.utils] Failed to
    compute_task_build_instances: ServerGroup policy is not supported:
      ServerGroupAffinityFilter not configured

This resets the global variables during base test setup, similar to how
other globals are reset.

Change-Id: Icbc75b1001c0a609280241f99a780313b244aa9d

blueprint: pci-device-tracking-in-placement
Change-Id: Id044131162e582cef41e424aa751e3cd3618ed0c

The stats module is used to decide if the InstancePCIRequests of a boot
request can fit to a given compute host and to decide which PCI device
pool can fulfill the requests. It is used both during scheduling and also
during the PCI claim code.

PCI devices now modelled in placement and the allocation_candidate query
now requests PCI resources therefore each allocation candidate returned
from placement already restricts which PCI devices can be used during
the PciPassthroughFilter, the NumaTopologyFilter, and PCI claim code
paths. This patch adapts the stats module to consider the PCI
allocation candidate or the already made placement PCI allocation when
filtering the PCI device pools.

blueprint: pci-device-tracking-in-placement
Change-Id: If363981c4aeeb09a96ee94b140070d3d0f6af48f

This patch extends the HostState object with an allocation_candidates
list populated by the scheduler manager. Also this changes the generic
scheduler logic to allocate the candidate of the selected host based on
the candidates in the host state.

So after this patch scheduler filters can be extended to filter the
allocation_candidates list of the HostState object while processing a
host and restrict which candidate can be allocated if the host passes
the all the filters. Potentially all candidates can be removed by
multiple consecutive filters making the host as a non viable scheduling
target.

blueprint: pci-device-tracking-in-placement
Change-Id: Id0afff271d345a94aa83fc886e9c3231c3ff2570

Fixed various small issues from the already merged (or being merged)
patches.

The logic behind the dropped FIXME and empty condition are handled in
the update_allocations() calls of the translator already.

The removal of  excutils.save_and_reraise_exception from the report
client is safe as self._clear_provider_cache_for_tree does not raise.

blueprint: pci-device-tracking-in-placement
Change-Id: If87dedc6a14f7b116c4238e7534b67574428c01c