Separate create server schema for admin password

admin password attribute in create server API's response
depend on the conf variable "enable_instance_password".

This patch separate create server schema based on this conf variable.

Partially implements blueprint nova-api-attribute-test

Change-Id: I897dd22dd5e130b4597b84c0e80eaed68c3e96a9
Merge "Separate create server schema for admin password"