Merge branch 'php-versions-api' into 'stable-3.1'

Remote API: new API call: server_get_php_versions.

This function was missing in a multi ISPC server environment, so I've implemented it. It has been tested and is working with both SOAP and REST calls. The result is an array of values that are accepted in the websites 'fastcgi_php_version' parameter.

See merge request !425