Allow user to specify instance resume behaviour on host reboot

[OpenStack Infra]

https://review.openstack.org/35710
commit ef2d85e579e9ab8fdffd29d51028cbbf64269068
Author: Phil Day <email address hidden>
Date: Thu Jul 4 21:17:30 2013 +0100

    Allow user to specify instance resume behaviour on host reboot

    The configuration option resume_guests_state_on_host_boot defines
    a global policy on whether or not to restart instances when a host
    reboots. Because a host may be down for a significant period of
    time many cloud applications monitor their own instances and launch
    replacements restarting all instances is rarely the required behaviour

    The blueprint provides the user with a mechanism to specify the
    resume policy on a per instance basis.

    Supported values are:
      Default - use system configuration value (current behaviour)
      Never - Don't resume this instance
      Always - Always resume this instance

    This is the first part of the blueprint which implements the resume
    decision. The API extension will be provided in a separate commit

    Code is structured to allow a futher option of "Resume only if host
    has been down for less than xxx seconds" - but we need support from
    the ServiceGroup API for that.

    Partly implements bp: user-defined-resume

    DocImpact

    Change-Id: I1f2969a9a32f8bf7c6b52ef76a7e2ac54e17554d