Allow disabling the evacuate cleanup mechanism in compute manager

[OpenStack Infra]

https://review.openstack.org/159890
commit 922148ac45c5a70da8969815b4f47e3c758d6974
Author: Dan Smith <email address hidden>
Date: Fri Feb 27 07:30:10 2015 -0800

    Allow disabling the evacuate cleanup mechanism in compute manager

    This mechanism attempts to destroy any locally-running instances on
    startup if instance.host != self.host. The assumption is that the
    instance has been evacuated and is safely running elsewhere. This is
    a dangerous assumption to make, so this patch adds a configuration
    variable to disable this behavior if it's not desired.

    Note that disabling it may have implications for the case where
    instances *were* evacuated, given potential shared resources.
    To counter that problem, this patch also makes _init_instance()
    skip initialization of the instance if it appears to be owned
    by another host, logging a prominent warning in that case.

    As a result, if you have destroy_after_evacuate=False and you start
    a nova compute with an incorrect hostname, or run it twice from
    another host, then the worst that will happen is you get log
    warnings about the instances on the host being ignored. This should
    be an indication that something is wrong, but still allow for
    fixing it without any loss. If the configuration option is disabled
    and a legitimate evacuation does occur, simply enabling it and then
    restarting the compute service will cause the cleanup to occur.

    This is added to the workarounds config group because it is really
    only relevant while evacuate is fundamentally broken in this way.
    It needs to be refactored to be more robust, and once that is done,
    this should be able to go away.

    DocImpact: New configuration option, and peril warning
    Partial-Bug: #1419785
    Change-Id: Ib9a3c72c096822dd5c65c905117ae14994c73e99