Use the new _get_content function to provide the notification content to
cURL in memory instead of making it read a file. This makes it possible
to use it together with DB storage.
This also relieves the notification poster from deleting the file if the
"purge" option is set, making it possible to simply roll it into file
output being disabled.
Retain legacy support for the "purge" option.
Change-Id: Ia1d59018d79777759de894aba1d888c927e5ba00