Check attributes of get server password Nova APIs

This patch adds the JSON schema for Nova V2 & V3 get server password
APIs response and validate the response with added JSON schema
to block the backward incompatibility change in the future.

The response body of get password V2 & V3 APIs is same and given below:

{
    "password": "%(encrypted_password)s"
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Iaec82632fd377435cdb10f8ac2e40c4fbd9f98f2
Merge "Check attributes of get server password Nova APIs"