Make LiveMigrateTask use build_request_spec()

Some filters like TrustedFilter makes use of extra_specs. Currently,
when live-migration uses the scheduler to select a host, it constructs
a request_spec that has no extra_specs in it.

By making use of the existing helper method build_request_spec(), the
request to the scheduler will include extra_specs.

Change-Id: I5bc6c6418653c256a42da7b0a343086ec9863da1
Closes-Bug: #1224014
Backported-from: icehouse
Upstream-Review: https://review.openstack.org/#/c/46735/
Closes-rally-bug: DE1096