Prevent reboot of rescued instance.

Do not allow rebooting an instance in the RESCUED state.  This will
revert the instance back into an ACTIVE state, which may still be
broken.  Require an explicit unrescue, instead.

Fix bug 1126163.

Change-Id: I144163666767923e3beb0a82e687066d1f3a0441
Merge "Prevent reboot of rescued instance."