Currently masakari delete's the host which doesn't belong to the
segment requested in the 'host delete' API's request URL because
host was not getting filtered on the basis of failover_segment_id.
Filtered the host based on 'failover_segment_id'. Added a new
exception 'HostNotFoundUnderFailoverSegment' which will be raised
from db API 'host_get_by_uuid' if the host is not found under the
requested failover_segment. This exception is not caught at API
controller layer as it is a subclass of 'HostNotFound' which is
already caught in controller.
Closes-Bug: #1697629
Change-Id: I3d3e8cffa2f8ea443ccd13b1db61fda55bc07a0d