Fixes evacuate doesn't honor enable password conf for v3

The evacuate api should honor enable_instance_password conf
setting just like server creation, rebuild and rescue api does,
enables this flag will returning the instance password from
server API call evacuate, If the hypervisor does not support
password injection then the password returned will not be
correct, this patch just fixes v3 evacuate api, since change
return value of v2 could potentially break apps which depend
on this being returned.

This patch also changes the evacuate response code to 202 to
keep consistency with api.

DocImpact
Change-Id: I2b9e5a067323ca84e2040e175a0425de3c0fafba
Closes-Bug: #1244203
Merge "Fixes evacuate doesn't honor enable password conf for v3"