Adds 'instance_type' param to build_request_spec

This change adds a new optional parameter to
scheduler.utils.build_request_spec. It also fixes Bug 1219761
by adding the new 'flavor' for cold migration instead of the
instance current one.

Change-Id: I28ac5f636e22e4d02ad5495b7b50c8226678c413
Merge "Adds 'instance_type' param to build_request_spec"