Add attachments API availability to /v1/systeminfo

Add a boolean representing whether or not attachments are enabled (and
therefore the API endpoints are available) to the /v1/systeminfo
endpoint. This allows clients to check whether attachments can be used
without having to have separate backend-specific configuration
themselves.

Change-Id: I949200d35e8591d1d670fd987ae7be7911aab983

storyboard/api/v1/system_info.py

@@ -43,6 +43,7 @@ class SystemInfoController(rest.RestController):
         """
         sb_ver = VersionInfo('storyboard-api')
         config = {
+            'enable_attachments': CONF.attachments.enable_attachments,
             'enable_editable_comments': CONF.enable_editable_comments,
             'enable_notifications': CONF.enable_notifications
         }