add tests for server_password

this adds tests for the following request:
GET /v2/{tenant_id}/servers/{server_id}/os-server-password
DELETE /v2/{tenant_id}/servers/{server_id}/os-server-password

this requires hypervisors or cloud_init script support, so
we just check the response code here rather than check the
content of the response

Change-Id: I641f7ca92fd7cb73df2f5b9cef34af53206ba8f7
Merge "add tests for server_password"