API Extension for Cloudbolt adding set-template-creds endpoint

[gamethis]

This Code will add an api extension to CloudBolt.
It will add the endpoint /api/v2/resource-handlers/{id}/Set-Template-Creds

that will take a payload of
POST: { "template": "{{template_name}}", "user-name": "{{user_name}}", "password": "{{password}}", "ssh-key":"{{ssh_key}}" }

[d-slingerland]

This PR needs more documentations, the use-case and implementation is not clear.

Please update the README with details about what the XUI intended use-case is, any implementation details and knowledge required for usage.