Prevent template validate from scanning ports

The template validation method in the heat API allows to specify the
template to validate using a URL with the 'template_url' parameter.

By entering invalid http URLs, like 'http://localhost:22' it is
possible to scan ports by evaluating the error message of the request.

For example, the request

curl -H "Content-Type: application/json" -H "X-Auth-Token: <TOKEN>" \
-X POST -d '{"template_url": "http://localhost:22"}' \
http://127.0.0.1:8004/v1/<TENANT_ID>/validate

causes the following error message to be returned to the user:

"Could not retrieve template: Failed to retrieve template:
('Connection aborted.',
BadStatusLine('SSH-2.0-OpenSSH_7.2p2 Ubuntu-4ubuntu2.1\\r\\n',))"

This could be misused by tenants to gain knowledge about the internal
network the heat API runs in.

To prevent this information leak, this patch alters the error message
to not include such details when the url scheme is not 'file'.

SecurityImpact

Closes-Bug: #1606500

Change-Id: Id1f86f41c1e6c028d889eca7ccbb9cde67631950

heat/common/urlfetch.py

@@ -75,4 +75,5 @@ def get(url, allowed_schemes=('http', 'https')):
         return result
 
     except exceptions.RequestException as ex:
-        raise URLFetchError(_('Failed to retrieve template: %s') % ex)
+        LOG.info(_LI('Failed to retrieve template: %s') % ex)
+        raise URLFetchError(_('Failed to retrieve template from %s') % url)