Add support for master-cluster

[erwindon]

Is your feature request related to a problem? Please describe. SaltStack 3007 introduced the "master-cluster" concept.

Describe the solution you'd like

• Build a master-cluster based environment
• Test SaltGUI with the master-cluster
• Adjust where appropriate, e.g. to better show the configuration for that cluster

see also https://saltproject.io/blog/salt-release-3007-is-now-available/ and https://github.com/saltstack/salt/blob/3007.x/doc/topics/tutorials/master-cluster.rst and https://github.com/saltstack/salt-enhancement-proposals/blob/1433501a1417f78c895345a675e21a8b6382bb61/0000-master-cluster.md