Validate nginx without relying on RestService

[mcouthon]

From @mcouthon on September 24, 2017 13:1

Currently the code that validates nginx is up and running requires the restservice to be installed, but restservice depends on rabbitmq, which in turn requires the certificates created in nginx. So we need to find an other way to validate it

Copied from original issue: mcouthon/cloudify-manager-install#40