bug 928521
modified nova/scheduler/manager.py to reset vm_state to ACTIVE and set
task_state to None when prep_resize raises a NoHostsFound
refactored run_instance and prep_resize so they don't go through
_schedule and now must be implemented in driver
Changed behavior to set vm_state to error on any other exception in
prep_resize.
Change behavior to change instance vm_state to ERROR on exceptions
Added tests that the vm_state gets updated
Added tests that schedule_prep_resize and schedule_run_instance
have no implementation in the Driver base class
Had to adjust methods and tests for Multi scheduler to reflect the
new Scheduler contract
Change-Id: Ibcac7ef0df3456793a2132beb7a711849510da80
8d1b4cee5b