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
Missing baremetal driver API test

As a part of blueprint, this patch implements following API test -
Driver APIs -
1. Retrieve a list of drivers.

'fake' driver is created by default during ironic installation.
For this please have a look on -
https://github.com/openstack/ironic/blob/master/setup.cfg#L33

part of blueprint: missing-baremetal-api-test

Change-Id: I5abe62f2fb9aec103ce4e127a28f488ef1f342df
Merge "Missing baremetal driver API test"
Merge "Check attributes of get server password Nova APIs"
Merge branch 'master' of https://github.com/openstack/tempest