Make gravatar hostname configurable

[kumy]

Description

Use alternative gravatar service like libravatar.org or using a self hosted avatar service.

Note

The module gravatar_image_tag used by Portainer doesn't support changing gravatar hostname yet. An issue exists there mdeering/gravatar_image_tag#34

When implemented it would be nice to permit configuring the module from the configuration file, not only the hostname, but also default_image (which can be actually monsterid, identicon, wavatar)