Separate scheduler host management

This is a bit of a scheduler refactoring to support future scaling work
as a part of blueprint scaling-zones.

Also fixes bug 891971 (remove unused set_network_host in scheduler)

With or without that blueprint work, this cleans a number of things up
and paves the way for combining a lot of the schedulers by using this
new 'HostManager' for filtering and weighing (future work :)

On to the goodies:

Introduces new HostManager, splitting code out from ZoneManager.
Zone communication and management is handlded in the ZoneManager.
Host filtering and weighing is handled in the HostManager.
ZoneManager is removed from the SchedulerManager and direct calls to it
  from the SchedulerManager now occur via the scheduler driver.  This
  simplifies a number of things.
ZoneManager and HostManager classes to use are now flags.  This allows
  one to extend the current classes and use them.
HostManager uses a HostState class to keep info about hosts.  This class
  needs to be extendable.  Since it's very much tied to the HostManager,
  the HostState class to use is not a flag.  It is, instead, a class
  variable in HostManager.
Filtering functions now accept a single host to filter.  This improves
  performance by not having to build a new array of hosts within every
  filter function.
Filtering functions now accept a more generic 'filter_properties'
  dictionary which we can fill with information available for filtering.
  Adding additional data to this 'filter_properties' can be done by
  subclassing.
Weighing functions now accept this 'filter_properties', also, although
  it's poorly named ATM.  Will be cleaned up in a future branch when I
  move some weighing functions into the host manager.
Filtering tests are true unit tests now.
test_zones was moved from top level to under scheduler as zone_manager
  tests and refactored to be true unit tests.
Host tests are true unit tests now.
Other minor cleanups

Change-Id: I0ef2acef6639b4500c400c18cf2c673cb80f0150

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