Server create - JSON schema validation: adminPass is optional

adminPass field is not returned to client if enable_
instance_password is set to False in nova.conf at Nova
API side. Thus it shouldn't be required by JSON schema
validation in Tempest.

Change-Id: I473a65bd87b493161e22edf39a7a07565007d8a9
Merge "Server create - JSON schema validation: adminPass is optional"