Reserved hosts can be shared between multiple host_aggregates. So
before evacuating the instances from failed_host to reserved_host,
the target resered_host should be added to the same aggregate in
which the failed_host is.
This patch adds the reserved_host to failed_host's aggregate.
Adding reserved_host to aggregate is optional and can be configured
by operators with the help of new configuration parameter
'add_reserved_host_to_aggregate' which is added under the 'host_failure'
section. This config option defaults to 'False'.
Change-Id: I7478e0f24ecd6fd6385dd67e7f0cad5ca3460526
d45f754cbb