Refactor MigrationTask._execute
Refactor MigrationTask._execute

This patch pulls out two functions from MigrationTask._execute. One for
the initial schedule case and the other for the re-schedule case. This
way unit testing some of the intricate parts of re-schedule becomes
easier.

Change-Id: I03b1eb1bd1a996081d1546134baa872c6166c5bb

openstack/nova,klmitch/nova,openstack/nova,openstack/nova,rahulunair/nova,rahulunair/nova,mahak/nova,klmitch/nova,klmitch/nova,mahak/nova,klmitch/nova,rahulunair/nova,mahak/nova