Allow custom icons for clusters

[joaquimrocha]

One nice way to quickly identify clusters is to assign different icons to them. So we should allow that as an option.

Acceptance Criteria:

• [ ] Offer a way for the user to configure a different icon for a cluster *

* TBD: Whether this will be a choice from a set of predefined icons, or a user uploaded icon, or both.

[blixtra]

A first step for this might be to generate an Identicon for the cluster. Here is an example of a project that does this in go: https://github.com/barthr/Identicon